Associated interferometers using multi-fiber optic delay lines

ABSTRACT

An interferometer apparatus which include two or more coupled fiber optic Michelson interferometers using fiber optic stretches which stretch two or more optical fibers wound around the perimeter of the optical fiber stretchers by the same amount is disclosed. Preferably a pair of reference and sample fiber optic stretches are utilized which run in a push-pull mode of operation. When one of the interferometers is a coherent light interferometer it can be used as a reference distance scale for all of the remaining low coherence light interferometer. A method for measuring a physical property of a device under test is also disclosed using the apparatus of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 14/529,263, filed on Oct. 31, 2014, the disclosure of which isincorporated herein by reference. This application is also related tocommonly owned U.S. patent application Ser. No. 13/794,577 filed Mar.11, 2013 and issued as U.S. Pat. No. 9,019,485 on Apr. 28, 2015; U.S.patent application Ser. No. 14/674,748 filed Mar. 31, 2015 and issued asU.S. Pat. No. 9,341,541 on May 17, 2016; and copending U.S. patentapplication Ser. No. 15/154,454 filed May 13, 2016, the disclosures ofwhich are incorporated herein by reference.

BACKGROUND Technical Field

This invention relates to fiber optic interferometers that incorporateoptical fiber stretchers, and more particularly to fiber opticinterferometers that incorporate optical fiber stretchers which stretchtwo or more fibers wound around them by the same amount.

Description of the Related Art

Optical fiber can be used to generate variable optical delays bycontrolling the length of a fiber loop. For example, multiple turns ofoptical fiber can be wound around a cylindrical piezo-electric (PZT)actuator under a sufficient tension which assures that the optical fibernever goes limp during operation. This forms a fiber optical delaydevice when an electrical voltage is applied to the cylindrical PZTactuator to cause the diameter of the cylindrical PZT actuator tochange. This change leads to a change in the circumference of thecylindrical PZT actuator and thus changes the extent of stretching onthe fiber loop, and thus the length of the optical fiber wound aroundthe actuator. This design of an optical fiber stretcher can be used toachieve fast delay variations as was described by Tearney et al. in anarticle entitled “Rapid acquisition of a in vivo biological images byuse of optical coherence tomography,” Optics Letters, Vol. 21, pp.1408-1410 (1996). The response speed of such a PZT-based fiber delaydevice can be fast, e.g., on the order of kHz or tens of kHz.Applications of these types of optical fiber stretchers are described inBush et al. in an article entitled “All-fiber optic coherence domaininterferometric techniques”, Proc. SPIE, Vol. 4204, pp. 71-80 (2001).

Other examples of fiber optic delay lines are described in U.S. Pat. No.5,835,642 by V. M. Gelikonov et al. entitled “Optical FiberInterferometer and Piezoelectric Modulator” issued on Nov. 10, 1998,which describes a variable fiber optic delay line formed of apiezoelectric plate with electrodes on both of its surfaces. (“Gelikonov'642” subsequently herein.) In this delay line an optical fiber iscoiled as a spiral and adhered to the piezoelectric plate on one or bothsurfaces of the plate. Applying an electric field to the piezoelectricplate results in a change in the diameter of the plate and hence thelength of the optical fiber adhered to the plate. U.S. Pat. No.5,867,268 by V. M. Gelikonov et al. entitled “Optical FiberInterferometer With PZT Scanning of Interferometer Arm Optical Length”issued on Feb. 2, 1999 describes interferometer configurations using thePiezoelectric Modulators which are described in Gelikonov '642.

U.S. Pat. No. 7,382,962 by S. Y. Xiaotian entitled “Fiber stretcherapparatus” issued on Jun. 3, 2008 describes various designs of fiberstretchers utilizing one or more linear actuators to stretch fiber loopsto create variable optical delay lines (“Xiaotian '962” subsequentlyherein). The fiber stretcher device described by Xiaotian '962 includesa stretcher frame that has a frame slot with a slot opening at one endto separate the frame into two parts that are connected at the other endof the frame slot. A linear actuator that expands or contracts along astraight line in response to a control signal produces a linear changein the dimension of the actuator along the straight line, and can bepositioned across the frame slot with one end fixed to one frame partand the other end fixed to the other frame part. The linear expansion orcontraction of the linear actuator exerts a force across the frame slotcausing the frame slot to expand or contract, acting like a spring. Thisdesign transforms a linear expansion or contraction of the actuator intoa change in the circumferential length of the stretch frame which can beshaped in various shapes having smooth surfaces including circles,ellipses, squares, and rectangles with rounded corners and racetrackshapes. This mechanism can be used to stretch a fiber loop formed bywinding optical fiber around the exterior surface of the stretcher framemultiple times under tension. U.S. Pat. No. 7,206,076 by T. Blalockissued on Apr. 17, 2007 entitled “Thickness Measurement of Moving Websand Seal Integrity System Using Dual Interferometer” (“Blalock '076”subsequently herein) also describes various designs of fiber opticstretchers.

Applications using optical fiber stretchers as fiber delay lines includelow coherence interferometry (LCI) and optical coherence tomography(OCT). LCI and OCT have applications in many fields from medical imagingto glass manufacturing. LCI and OCT have been adapted to the non-contactmeasurement of physical properties of various materials includingthickness, index of refraction measurement, surface profiles, and indexof refraction profiles. LCI and OCT can also be used to measuredistances between optical surfaces of a test structure. The LCItechnique is based on using a light source with a short coherencelength. The light is split between two arms or branches of aninterferometer and then recombined and directed onto a detector.Interference occurs when the path lengths of light reflecting off ofobjects located in the two branches of the interferometer are equal towithin the coherence length of the light from the source.

There are numerous known configurations of such interferometers, such asthe Michelson, Mach-Zehnder, and Fizeau interferometers, and othersdescribed in the text, Principles of Optics: Electromagnetic Theory OfPropagation, Interference and Diffraction of Light, M. Born and E. Wolf,Cambridge University Press, Cambridge; New York, 1999, 7th ed. Anotherexample of such an interferometer is described in U.S. Pat. No.6,724,487 of M. A. Marcus et al. issued on Apr. 20, 2004 entitled,“Apparatus and method for measuring digital imager, package and waferbow and deviation from flatness,” the disclosure of which isincorporated herein by reference. (“Marcus '487” subsequently herein.)

The interferometer disclosed therein by Marcus '487 as shown in FIG. 9of Marcus '487 is based on the use of piezo fiber stretching technologyas the means of changing the optical path-length in the two arms of theinterferometer. A narrow beam of low-coherence light is directed ontothe surface of the device under test. It is common to focus the beaminside or in proximity to the device under test. The reflected lightfrom all of the optical interfaces of the device under test, which thebeam traverses, is then combined with light from a coherent light sourceof a distinct wavelength using a wavelength division multiplexer (WDM).The combined light passes through a 50/50 fiber coupler into a pair ofsingle mode optical fibers that are coiled around a pair ofpiezoelectric cylinders which make up the two arms of the fiberMichelson interferometer. Voltages are applied to the piezoelectriccylinders in a push-pull mode to alternately change the optical pathlength of light being transmitted along the optical fibers wound aroundthe cylinders. Reflectors are located at the ends of the optical fibersafter the coils to send the light back through the fiber coils.Interfering light returning from the interferometer arms is sent toseparate low coherence and coherent light detectors. The coherent lightinterferometer provides a built in distance scale and is used to triggerdata acquisition from the low coherence source at constant distanceintervals. The low coherence interferometer is used to determine theoptical distances between the interfaces in the device under test. Thephysical distances are obtained by dividing the optical distances by thegroup refractive indices of the material which makes up the spacebetween the interfaces.

Other designs of dual interferometers in which a coherent light sourceis used together with a low coherence light source are described in U.S.Pat. No. 5,596,409 issued on Jan. 21, 1997 entitled “Associated DualInterferometric Measurement Method for Determining a Physical propertyof an Object” by M. A. Marcus and S. Gross (“Marcus '409” subsequentlyherein), U.S. Pat. No. 5,659,392 issued on Aug. 19, 1997 entitled“Associated Dual Interferometric Measurement Apparatus for Determining aPhysical property of an Object” by M. A. Marcus et al. (“Marcus '392”subsequently herein), “Blalock '076,” and U.S. Pat. No. 6,847,453 issuedon Jan. 25, 2005 entitled “All Fiber Autocorrelator” by I. J. Bush.“Marcus '409” and “Marcus '392” describe how to use the coherent lightinterferometer as a distance scale by sampling at zero crossings of thecoherent light interferogram and using the zero crossings to sample thelow coherence light interferogram at constant distance intervals. Thisapproach is called distance based sampling as opposed to the traditionalapproach of time based sampling.

In previous designs of dual interferometers using optical fiberstretchers as optical delay lines, the coherence source and the lowcoherence source operate at different wavelengths. The coherent lightand low coherence light are combined using a WDM and are sent down asingle fiber which changes path length while being transmitted along thefiber delay line. Optical fibers undergo dispersion effects whenstressed and strained and different wavelengths of light have differentcoefficients of dispersion as a function of temperature and stress. Thiscan cause changes in the calibration of the distance scale of theinterferometer as a function of optical fiber stretcher temperaturewhich can result in measurement errors.

Cost of components in building interferometers is also an importantparameter to manufacturers of such instruments. Anything that can bedone to reduce the cost of manufacturing by eliminating expensiveoptical components is desirable.

The disclosures of all of the aforementioned patents are incorporatedherein by reference. The disclosures of these patents notwithstanding,there remains an unmet need for a more precisely controlled fiber delayline measurement apparatus and method that eliminates temperaturedependent differential dispersion effects to provide a more precisecalibration. There is also a need for a more precisely controlled lowcoherence interferometer to provide improved measurement reproducibilityon measurements of physical parameters of test objects at a lower cost.

SUMMARY

The present invention meets this need by providing an apparatus whichincludes one or two optical fiber stretchers which simultaneously varythe length of two or more optical fibers which are wound together aroundthe outer surface of the optical fiber stretcher by the same amount.These optical fiber stretchers are then incorporated into coupledinterferometers which share the common optical fiber stretchers. Whenlight of the same wavelength distribution is sent through each of two ormore optical fibers which are wound together around the outer surface ofthe optical fiber stretcher each of the interferometers will have thesame optical path length differences when the optical fiber stretchersare actuated.

In a first embodiment of this invention an interferometer apparatus isprovided comprising a reference optical fiber stretcher comprising areference outer surface defining a reference perimeter having first andsecond reference delay optical fibers wound around the reference outersurface, and a reference actuator configured to temporally vary thereference perimeter of the reference outer surface. The interferometerapparatus also comprises a sample optical fiber stretcher comprising asample outer surface defining a sample perimeter having first and secondsample delay optical fibers wound around the sample outer surface, and asample actuator configured to temporally vary the sample perimeter ofthe sample outer surface. The interferometer apparatus also comprises afirst fiber optic coupler which receives coherent light of wavelengthλ_(c) from a coherent light source and transmits a first portion of thecoherent light into an input end of the first reference delay opticalfiber and transmits a second portion of the coherent light into an inputend of the first sample delay optical fiber. The interferometerapparatus further comprises a second fiber optic coupler which receivesfirst low coherence light of center wavelength λ₁ from a first lowcoherence light source and transmits a first portion of the first lowcoherence light into an input end of the second reference delay opticalfiber and transmits a second portion of the first low coherence lightinto an input end of the second sample delay optical fiber. Theinterferometer apparatus also comprises a first reference reflectorcoupled to an output end of the first reference delay optical fiberwhich reflects coherent light back through the first reference delayoptical fiber, and back through the first fiber optic coupler into acoherent light detector; and a first sample reflector coupled to anoutput end of the first sample delay optical fiber which reflectscoherent light back through the first sample delay optical fiber, andback through the first fiber optic coupler and into the coherent lightdetector. The interferometer also comprises a second reference reflectorcoupled to an output end of the second reference delay optical fiberwhich reflects first low coherence light back through the secondreference delay optical fiber, and back through the second fiber opticcoupler into a first low coherence light detector. The interferometerapparatus further comprises a first optical probe coupled to the outputend of the second sample delay optical fiber and is configured totransmit first low coherence light to a first location on an objectcomprising at least one optical interface, and receive first lowcoherence light reflected back from the at least one optical interface,and transmit the reflected first low coherence light back through thefirst optical probe, back through the second sample delay optical fiber,and back through the second fiber optic coupler and into the first lowcoherence light detector.

In a second embodiment of this invention an apparatus is providedcomprising m associated interferometers where m is an integer greaterthan 1, where each of the m associated interferometers have a referencebranch and a sample branch and each of the m associated interferometersfurther comprise a common reference optical fiber stretcher comprising areference outer surface defining a reference perimeter, m referencedelay optical fibers wound around the reference outer surface, and areference actuator configured to temporally vary the perimeter of thereference outer surface. Each of the m associated interferometers alsocomprise a common sample optical fiber stretcher comprising a sampleouter surface defining a sample perimeter, m sample delay optical fiberswound around the sample outer surface, and a sample actuator configuredto temporally vary the perimeter of the sample outer surface. Each ofthe m associated interferometers also comprise a fiber optic couplerwhich receives coherent or low coherence light from a coherent or lowcoherence light source and transmits a first portion of the coherent orlow coherence light into an input end of a distinct one of the mreference delay optical fibers and transmits a second portion of thecoherent or low coherence light into an input end of a distinct one ofthe m sample delay optical fibers. In particular the j^(th) fiber opticcoupler, where j is an integer with 1≤j≤m, receives coherent light ofwavelength λ_(c) from a coherent light source and transmits a firstportion of the coherent light into an input end of the j^(th) referencedelay optical fiber and transmits a second portion of the coherent lightinto an input end of the j^(th) sample delay optical fiber, and theremaining m−1 fiber optic couplers receive low coherence light from alow coherence light source and transmit a first portion of the lowcoherence light into an input end of the corresponding remaining one ofthe m−1 reference delay optical fibers and transmit a second portion ofthe low coherence light into an input end of the corresponding remainingone of the m−1 sample delay optical fiber. Each of the m associatedinterferometers also comprise a reference reflector coupled to an outputend of each of the distinct one of the m reference delay optical fiberswhich reflects coherent or low coherence light back through the distinctone of the m reference delay optical fibers and back through thecorresponding distinct one of the m fiber optic couplers and into acorresponding detector. A sample reflector is coupled to the output endof the j^(th) sample delay optical fiber which reflects coherent lightback through the j^(th) sample delay optical fiber and back through thej^(th) fiber optic coupler and into the j^(th) detector. An opticalprobe is coupled to the output end of each of the distinct one of theremaining m−1 sample delay optical fibers, and is configured to transmitlow coherence light to a location on an object comprising at least oneoptical interface, and to receive low coherence light reflected backfrom the at least one optical interface, and transmit the reflected lowcoherence light back through the optical probe, back through thedistinct one of the remaining m−1 sample delay optical fibers, and backthrough the corresponding distinct one of the m−1 fiber optic couplersand into the corresponding detector.

These and other aspects, objects, features and advantages of the presentinvention will be more clearly understood and appreciated from a reviewof the following detailed description of the preferred embodiments andappended claims, and by reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be provided with reference to the followingdrawings, in which like numerals refer to like elements, and in which:

FIG. 1A shows a schematic of a first embodiment of an optical fiberstretcher used to stretch two optical fibers by the same amount.

FIG. 1B shows a schematic of an embodiment of an optical fiber stretcherused to stretch three optical fibers by the same amount.

FIG. 1C shows a schematic of an embodiment of an optical fiber stretcherused to stretch four optical fibers by the same amount.

FIG. 2A shows a schematic of a first associated two optical fiberinterferometer embodiment of the present invention.

FIG. 2B shows a schematic of a second associated two optical fiberinterferometer embodiment of the present invention.

FIG. 2C shows a schematic of a third associated two optical fiberinterferometer embodiment of the present invention.

FIG. 3A shows a schematic of a first associated three optical fiberinterferometer embodiment of the present invention.

FIG. 3B shows a schematic of a second associated three optical fiberinterferometer embodiment of the present invention.

FIG. 4A shows a schematic of a first associated two optical fiberautocorrelator embodiment of the present invention.

FIG. 4B shows a schematic of a second associated two optical fiberautocorrelator embodiment of the present invention.

FIG. 5A shows a schematic of a first associated three optical fiberautocorrelator embodiment of the present invention.

FIG. 5B shows a schematic of a second associated three optical fiberautocorrelator embodiment of the present invention.

FIG. 6A shows a schematic of a first associated three optical fiberhybrid interferometer embodiment of the present invention.

FIG. 6B shows a schematic of a second associated three optical fiberhybrid interferometer embodiment of the present invention.

FIG. 7A shows a schematic of a first associated m optical fiberinterferometer embodiment of the present invention.

FIG. 7B shows a schematic of a second associated m optical fiberinterferometer embodiment of the present invention.

FIG. 8A shows an example reference test object which could be used forcalibration of associated m optical fiber interferometers.

FIG. 8B shows an interferometer depth scan of the reference test objectshown in FIG. 8A.

FIG. 9 shows a graph of piezoelectric actuator voltage waveform versustime and the change in optical path difference versus time for referenceand sample actuators of a Michelson interferometer operating in apush-pull mode.

FIG. 10 shows a flow chart detailing the steps used for measuring aphysical property of an object according to an embodiment of the presentinvention.

FIG. 11 shows an example coherent light interference signal for a 1310nm laser diode.

FIG. 12 shows an interferogram for an object along with the optical pathdifference between the two branches of the interferometer as a functionof cumulative scan distance.

FIG. 13 shows a schematic of an alternate embodiment of an associatedtwo optical fiber interferometer embodiment of the present invention.

The present invention will be described in connection with preferredembodiments; however, it will be understood that there is no intent tolimit the invention to the embodiments described. On the contrary, theintent is to cover all alternatives, modifications, and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

DETAILED DESCRIPTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance tothe invention. For a general understanding of the present invention,reference is made to the drawings. It is to be understood that elementsnot specifically shown or described may take various forms well known tothose skilled in the art. In the following description and drawings,identical reference numerals have been used, where possible, todesignate identical elements.

The example embodiments of the present invention are illustratedschematically in order to illustrate key principles of operation of thepresent invention and are not drawn with intent to show actual size orscale. Some exaggeration, i.e., variation in size or scale may benecessary in order to emphasize relative spatial relationships orprinciples of operation. One of ordinary skill in the art will be ableto readily determine the specific size and interconnections of theelements of the example embodiments of the present invention.

In the following disclosure, the present invention is described in thecontext of its use as a variable multiple fiber optical delay line andas an interferometer to measure one or more physical properties of adevice under test which incorporates one or two of the variable multiplefiber optical delay lines. In the context of the present disclosure, adevice under test is considered to be an object which comprises at leastone optical interface that is measured at one or more measurementlocations. Additionally, this description may identify certaincomponents with the adjectives “top,” “upper,” “bottom,” “lower,”“left,” “right,” “horizontal”, “vertical”, “inner”, “outer” etc. Theseadjectives are provided in the context of use of the apparatus as ameasurement device, and in the context of the orientation of thedrawings, which is arbitrary. The description is not to be construed aslimiting the apparatus to use in a particular spatial orientation. Theinstant apparatus may be used in orientations other than those shown anddescribed herein.

The present invention is also described in terms of optical fibercomponents being coupled together. In the context of the presentinvention the words “coupled to” imply that light travels from onecomponent to the other, either via an optical fiber or through air.Other components not specifically called out can also be interposedbetween components that are coupled together without changing the factthat the components described are coupled components. As an example, thedescription of a component (A) being coupled to a second component (B)is not to be construed as limiting the apparatus to have the components(A) and (B) being directly coupled to each other. Component (A) may becoupled to another component (C) which is not called out in thedescription and then component (C) may be coupled to component (B).Multiple uncited components may also be added between coupled componentsand not change the fact that the original two components are stillcoupled together.

Various embodiments of multi-optical fiber variable optical delay lines,also called optical fiber stretchers or fiber optic stretchers of thepresent invention for simultaneously varying the length of two or moreoptical fibers by a fixed proportion are shown in FIG. 1A-FIG. 1C. Themain components of the optical fiber stretching apparatuses are theoptical fiber stretcher, an actuator and two or more optical fiberswrapped or wound around the perimeter of the optical fiber stretcher.When single mode optical fibers are utilized, the optical path lengthfor light propagating or being transmitted along an individual opticalfiber is directly proportional to the length of that individual opticalfiber. Throughout this disclosure, the notation “delay optical fiber”will denote an optical fiber that is wound around the perimeter of anoptical fiber stretcher. In a delay optical fiber, the optical path oflight transmitted along the delay optical fiber will change as theperimeter of the optical fiber stretcher is varied with time.

FIG. 1A shows a first embodiment of an optical fiber stretcher 10 havinga uniform outer surface defining a uniform perimeter comprising twodelay optical fibers being wound around the uniform outer surface of theoptical fiber stretcher 10. It is preferred that the lengths of the twooptical fibers in contact with the perimeter of the optical fiberstretcher are the same and that they are wound together onto the opticalfiber stretcher in a single layer so that they will stretch by the sameamount as the perimeter of the optical fiber stretcher is temporallyvaried. The optical fiber stretcher also includes an actuator configuredto temporally vary the perimeter of the optical fiber stretcher. In theembodiment shown, the optical fiber stretcher 10 is shown as apiezoelectric cylinder 5 having an outer diameter d, height h and wallthickness t with an inner surface 7 and an outer surface 6. Thepiezoelectric cylinder serves as both the stretching frame and theactuator. An important property of a cylinder is that it has a uniformcircumference defined as πd. The term circumference is defined as theperimeter of a circle and perimeter is defined as the length of a closedcurve enclosing an area. Since the perimeter of a circular orcylindrical object is the same as circumference, the more general termperimeter is used throughout this disclosure since the fiber opticstretcher may not be cylindrical in shape. First delay optical fiber 1(dashed line) and second delay optical fiber 2 (solid line) are shownwound continuously around the piezoelectric cylinder's outer surface 6in a single layer. The optical fibers 1 and 2 are called delay opticalfibers since they are the optical fibers which form the fiber opticdelay line when they are stretched. The total height of the opticalfibers wound around the piezoelectric cylinder is defined as h_(w) asshown in FIG. 1A.

The optical fiber stretcher 10 is preferably designed to simultaneouslyvary the length of first and second delay optical fibers 1 and 2 by thesame amount. The optical fiber stretcher 10 has a uniform outer surfacedefining a uniform perimeter and the first and second delay opticalfibers 1 and 2 are wound together around the outer surface 6 of theoptical fiber stretcher 10 in a single layer. The optical fiberstretcher 10 also comprises an actuator configured to temporallyuniformly vary the perimeter of the optical fiber stretcher. Thisinsures that each loop of optical fiber wound around the perimeter ofthe fiber optic stretcher will stretch by the same amount. In apreferred embodiment of the optical fiber stretcher 10, delay opticalfibers 1 and 2 are interleaved with each other which creates aninterleaved arrangement also called an interleave pattern (see Table 1below) as they are wound around the piezoelectric cylinder's outersurface 6 in a single layer. It is also preferred that the lengths ofthe sections of the delay optical fibers 1 and 2 being interleaved andwound together around the outer surface 6 of the sample optical fiberstretcher 10 in a single layer are the same. When light of the samewavelength is transmitted along delay optical fibers 1 and 2, they willundergo the same optical path length change as the perimeter of theoptical fiber stretcher is varied as a function of time. Furthermore, itis also preferred that all adjacent interleaved delay optical fibers arein contact with each other when they are wound together around the outersurface 6 of the sample optical fiber stretcher 10 in a single layer.This maximizes the amount of optical fiber that can be wound around theoptical fiber stretcher 10 in a single layer.

FIG. 1B shows a second embodiment of an optical fiber stretcher 13having a uniform outer surface 6 defining a uniform perimeter comprisingthree delay optical fibers being wound together around the outer surface6 of the optical fiber stretcher 13 in a single layer for simultaneouslychanging the length of three delay optical fibers by a fixed proportion.The optical fiber stretcher 13 shown in FIG. 1B includes all of thecomponents shown in FIG. 1A, with the addition of a third delay opticalfiber 3 (dotted lines) being interleaved together with first and seconddelay optical fibers 1 and 2 during winding. In the preferred embodimentthe length of all three delay optical fibers 1, 2, and 3 woundcontinuously around the piezoelectric cylinder's outer surface are thesame, and all adjacent interleaved delay optical fibers are in contactwith each other.

FIG. 1C shows a third embodiment of an optical fiber stretcher 14 havinga uniform outer surface 6 defining a uniform perimeter comprising fourdelay optical fibers being wound together around the outer surface ofthe optical fiber stretcher 14 in a single layer for simultaneouslychanging the length of four delay optical fibers by a fixed proportion.The optical fiber stretcher 14 shown in FIG. 1C includes all of thecomponents shown in FIG. 1B with the addition of a fourth delay opticalfiber 4 (dot-dashed lines) being interleaved together with delay opticalfibers 1, 2, and 3 during winding. In the preferred embodiment, thelength of all four delay optical fibers 1, 2, 3, and 4 woundcontinuously around the piezoelectric cylinder's outer surface are thesame, and all adjacent interleaved delay optical fibers are in contactwith each other.

More delay optical fibers may also be wound together around a singleoptical fiber stretcher as required for the particular application.Table I shows the interleaved arrangement also called the interleavepattern used versus the number of fibers being interleaved together inorder to have all of the fibers that are wound together around a singleoptical fiber stretcher to have substantially the same length. The firstcolumn shows the number of fibers being wound together around theuniform perimeter of the optical fiber stretcher and the second columnshows the interleaved arrangement. Letters A, B, C, D, E and F refer todelay optical fibers 1, 2, 3, 4, 5, and 6 respectively. The last rowshows the interleaved arrangement for an optical fiber stretcher havingm delay optical fibers where m is an integer greater than or equal to 2.

TABLE I Interleaved Arrangement vs Number of Delay Optical Fibers WoundTogether # fibers Interleave Pattern 2 ABABABABABABABABABABABAB . . .ABABABABABABABABABAB 3 ABCABCABCABCABCABCABCABC . . .ABCABCABCABCABCABCABC 4 ABCDABCDABCDABCDABCDABCD . . .ABCDABCDABCDABCDABCD 5 ABCDEABCDEABCDEABCDE . . . ABCDEABCDEABCDEABCDE 6ABCDEFABCDEFABCDEFABCDEF . . . ABCDEFABCDEFABCDEFABCDEF m AB . . . mAB .. . mAB . . . mAB . . . mAB . . . m . . . AB . . . mAB . . . mAB . . .mAB . . . m

During winding around the piezoelectric cylinder 5, the delay opticalfibers 1 to m are preferably wound together in an interleavedarrangement while under constant tension during the winding process. Thedelay optical fibers are also preferably wound in a single layer. It isalso preferred that each delay optical fiber be in contact with eachadjacent delay optical fiber during winding. When adjacent interleavedfibers are in contact with each other, the distance between the centersof adjacent fibers is equal to the outer diameter of the optical fibers.The delay optical fibers are usually communication grade optical fibershaving a core, cladding and a buffer coating. For communication gradeoptical fibers the outer surface of the optical fiber would be the outersurface of the buffer coating. Having the outer surfaces of adjacentdelay optical fibers be in contact with each other maximizes the totallength of optical fiber that can be wound around the cylinder 5 in asingle layer and helps ensure that the tension is uniform across theentire optical fiber stretcher surface 6. This results in having thetotal lengths of all of the delay optical fibers wound around thecylinders being substantially the same length. After winding, the delayoptical fibers are bonded to the piezoelectric cylinder with a thinlayer of adhesive, typically an epoxy adhesive. Bonding of the delayoptical fibers to the piezoelectric cylinder has a negligible effect onthe stretching and relaxation of the optical fiber during actuation ofthe piezoelectric cylinder

The inner surface 7 and outer surface 6 of the piezoelectric cylinder 5are both electroded. When a voltage V is applied across the electrodesurfaces of piezoelectric cylinder 5 having a diameter d much largerthan its thickness t, the diameter d of the piezoelectric cylinder 5changes as a function of applied voltage by the relationship

${\Delta\; d} = {d_{13}d\frac{V}{t}}$where d₃₁ is the appropriate piezoelectric strain coefficient of thepiezoelectric material in the 31 direction, i.e., the induced strain indirection 1 per unit electric field applied in direction 3, wheredirections 1, 2, and 3 are defined using standard x-y-z right-handedcoordinate axes. (Direction 1 is the x axis, and direction 3 is the zaxis). Applying a voltage across the electrode surfaces of thepiezoelectric cylinder 5 thus introduces strain on the delay opticalfibers wound around the piezoelectric cylinder which causes the diameterof the delay optical fibers to change as a function of applied voltage.The position of the center of the core of the optical fiber is used asthe reference for defining the location of the optical fiber. Theeffective diameter of a single loop of optical fiber wound around theouter surface of the piezoelectric cylinder is thus given by d+d_(f) andthe length of delay optical fiber L_(f) that can be wound around apiezoelectric cylinder having an outer diameter d is given by therelationship

$L_{f} = {{\pi\left( {d + d_{f}} \right)}\frac{h_{w}}{d_{f}}}$where d_(f) is the outer diameter of the delay optical fiber. The totalchange in optical path length (ΔOPL) of the total amount of delayoptical fiber being wound around the piezoelectric cylinder in a singlelayer when a voltage V is applied to the piezoelectric cylinder is givenby the relationship

${\Delta\;{OPL}} = {n_{c}{\pi\Delta}\; d\frac{h_{w}}{d_{f}}}$where n_(c) is the index of refraction of the core of the delay opticalfiber at the wavelength of the light being transmitted along the delayoptical fiber. When m delay optical fibers having outer diameter d_(f)are interleaved and wound together around a single piezoelectriccylinder of diameter d so that the outer surface of each delay opticalfiber is in contact with the outer surface of each adjacent interleavedoptical fiber, the change in optical path of each individual fiber(ΔOPL_(f)) is given by

${\Delta\;{OPL}_{f}} = {\frac{\Delta\;{OPL}}{m}.}$

In the preferred embodiments it is desired that the lengths of all ofthe delay optical fibers wound around the piezoelectric cylinder are thesame and that all of the delay optical fibers are made of the samematerial having the same form factor. In this case the length of all ofthe delay optical fibers will change by the same amount as the perimeterof the optical fiber stretcher is varied uniformly. In preferredembodiments the delay optical fibers are single mode optical fibers orpolarization maintaining optical fibers.

Although the optical fiber stretchers shown in FIG. 1A-FIG. 1C arepiezoelectric cylinders, it is possible to replace the piezoelectriccylinder stretchers with other types of stretching elements or stretcherframes having uniform perimeters. Other example stretcher frames includethe race track oval type stretcher frame described in Xiaotian '962, orthe various designs described in Blalock '076.

The multi-optical fiber variable optical delay lines described in FIG.1A-1C can be used to create variable optical path delay elements ininterferometers as described below. FIG. 2A-FIG. 7B show schematics ofvarious associated multiple optical fiber interferometer embodiments ofthe present invention. These various embodiments are used to measure oneor more physical properties of a device under test also called an objecta 42 in the Figures.

The term physical properties as used in this application refer to bothoptical properties (for example index of refraction) and the physicaldimensions (such as thickness and surface shape) as well as combinationsof the two (such as optical thickness) of a test object. Physicalproperties of the object (also called device under test) may alsoinclude optical distances between adjacent layers of the object.Distances between a reference surface and the object can also beconsidered to be physical properties of the object. Throughout thisdisclosure, we use the word object to mean a test object comprising atleast one optical interface. Surface and thickness profiles of theobject can be obtained by using transport stages coupled to the opticalprobe or the object. Other physical properties of the object can bemeasured by varying a property of the environment such as pressure ortemperature. As an example the coefficient of thermal expansion of theobject can be measured by determining the physical thickness as afunction of temperature.

All of the interferometer embodiments of the present invention includeat least two optical fiber interferometers which are associated togetherby sharing one or two common optical fiber stretching elements which areused to change the path lengths of the respective branches of the atleast two interferometers by the same proportion and preferably by thesame amount. In FIG. 2A-FIG. 7B, optical fibers which couple adjacentcomponents together are shown as having various line types includingdashed lines, solid lines, dotted lines, or dotted-dashed lines. Allcomponents shown as being coupled together by optical fibers having thesame line type are components of the same interferometer.

In FIG. 2A-FIG. 7B, suffixes on component names (-1, -2, -3, -j, -m)indicate which of the individual associated interferometers (1, 2, 3, j,m) that the component is a part of. The embodiments of the presentinvention shown in FIG. 2A—FIG. 6B and FIG. 7B combine a laserinterferometer which is used as a reference distance scale with at leastone low coherence interferometers. The combination of a laserinterferometer with a low coherence interferometer is called a dualinterferometer as disclosed in “Marcus '392” and “Marcus '487”. In theFigures, the associated coherent light interferometer is configured inthe standard mode Michelson interferometer configuration. The associatedlow coherence interferometers are configured in either the standard modeMichelson configuration or in the autocorrelation mode in which thedevice under test is located at the input to a standard Michelsoninterferometer.

By convention, in a standard mode Michelson interferometer, a reflectoris located in the first branch of the Michelson interferometer, and thesample or device under test, which we also call the object, is locatedin the second branch of the Michelson interferometer. In anautocorrelation mode Michelson interferometer, light that is sent to thedevice under test is reflected back from optical interfaces of thedevice under test into a Michelson interferometer having reflectors inboth the first second branches of the interferometer. Throughout thedisclosure of the present invention we will use the prefixes “reference”and “sample” on component names to define which branch of theinterferometer that the component is located in. The prefix “reference”in a component name indicates that the component is in the first branchof the interferometer and the prefix “sample” indicates that thecomponent is in the second branch of the interferometer. In the case ofthe autocorrelator mode the use of the term “sample branch” does notimply that the device under test is located in that branch.

When referring to optical fiber stretchers throughout this disclosure wewill use the term reference optical fiber stretcher and sample opticalfiber stretcher to denote the optical fiber stretchers located in thereference and sample branches of the interferometer respectively. In theembodiments of this invention, the reference and sample optical fiberstretchers have outer surfaces with uniform perimeters which we refer toas the reference outer surface and the sample outer surfacerespectively. Thus, throughout this disclosure the term reference outersurface defines a reference perimeter which is the uniform perimeter ofthe reference outer surface, and the term sample outer surface defines asample perimeter which is the uniform perimeter of the sample outersurface.

FIG. 2A-FIG. 3B show schematics of various configurations of lowcoherence standard mode Michelson interferometer embodiments of thisinvention while FIG. 4A-FIG. 5B show various configurations of lowcoherence autocorrelation mode Michelson interferometer embodiments ofthis invention. FIG. 6A and FIG. 6B show schematics of variousconfigurations of hybrid low coherence interferometer embodiments ofthis invention which include at least one low coherence standard modeMichelson interferometer and at least one low coherence autocorrelationmode Michelson interferometer associated together.

When using the autocorrelator mode Michelson interferometerconfigurations shown in FIG. 4A-FIG. 6B, a reference reflection may beadded inside the optical probe that is coupled to the device under test.The reference reflector is set up at a known distance from the opticalprobe focal depth on the device under test and the zero voltage opticalpath difference of the two branches of the Michelson interferometer areoffset by an amount close to this known distance. This allows theinterferogram obtained as a function of scan depth to appear the same asin the standard mode interferometer.

In FIG. 2A-FIG. 7B optical fibers are indicated by dashed lines(interferometer 1), solid lines (interferometer 2), dotted lines(interferometer 3) and dotted-dashed lines (interferometer m). Theindividual fibers which connect components are not all labelledindividually, but it is to be understood that optical fibers couplelight from the input component into the output component. The input andoutput ends of the components are indicated by solid circles. Light iscoupled into the optical fibers from the component at the input end andis transmitted along the optical fiber and is then coupled into thecomponent at the output end. When light is reflected from a downstreamcomponent, light will propagate back down the optical fiber in theopposite direction. Also, the solid circles at the input and output endsof the components do not imply that there is a fiber termination locatedat each component end as the various components may contain fiberpigtails or exposed optical fibers at both ends. Optical fiberstretchers 10R, 10S, 13R, 13S, 16R and 16S are examples of componentswhich may contain fiber pigtails and exposed optical fibers at bothends. The optical fibers may also contain splices or optical fiberconnectors between the various components which are not shown in thefigures. Other components including filters, polarizers, couplers,wavelength division multiplexers (coupled to other light sources) andattenuators may also be added between components that are shown in theFigures.

All of the interferometer embodiments of this invention contain variouslight sources and detectors. The light sources may be coherent or lowcoherence light sources and may operate at a variety of wavelengths. Thedetectors are used to detect interferograms arising from interferencebetween two beams of light produced by the interferometer. The detectormay be any kind of suitable detector including a photodiode, avalanchephotodiode, or a balanced detector. The same detector may be used todetect coherent light and low coherence light.

In the standard mode Michelson interferometers shown in FIG. 2A-FIG. 3Band FIG. 7A-FIG. 7B, the object 42 is located in the sample branch ofthe interferometers while in the autocorrelation mode interferometersshown in FIG. 4A-FIG. 5B the light is incident on the device under testand reflected light coming from the optical interfaces in the deviceunder test is input into the reference and sample branches of theMichelson interferometers. The various optical fiber stretchers 10R,10S, 13R, 13S, 16R, and 16S are common to each of the associatedMichelson interferometers shown in each of the respective figures andare used to vary the optical path length of light of the same wavelengthbe transmitted through all of the optical fibers located in the branchesof each of the Michelson interferometers by the same amount. Capitalbold letter R and S indicate the reference and sample branches of theinterferometer. R and S suffixes on component numbers are used todifferentiate components which occur in both branches of theinterferometer and indicate that the component is located in thereference branch or the sample branch respectively. The numbers used torepresent optical fibers in the optical fiber stretchers (1R, 1S, 2R,2S, 3R, 3S, mR, mS) making up the optical fiber delay lines are also thesame as the distinct associated interferometer number in the figures.

In FIG. 2A-FIG. 3B, all of the components connected together by dashedlines form a coherent light Michelson interferometer in the standardmode configuration; all of the components connected together by solidlines form a first low coherence Michelson interferometer in thestandard mode configuration; and all of the components coupled togetherby dotted lines form a second low coherence Michelson interferometer ina standard mode configuration. In FIG. 4A-FIG. 6B, all of the componentscoupled together by dashed lines form a coherent light Michelsoninterferometer in a standard mode configuration, and all of thecomponents coupled together by solid lines form a first low coherenceinterferometer operating in an autocorrelation mode. In FIG. 4A-FIG. 5B,all of the components coupled together by dotted lines form a second lowcoherence interferometer operating in an autocorrelation mode. Howeverin FIG. 6A-FIG. 6B, all of the components coupled together by dottedlines form a second low coherence Michelson interferometer configured inthe standard mode. The interferometer configurations shown in FIG.6A-FIG. 6B are considered hybrid configurations since there are twoseparate associated low coherence interferometers which operate indifferent modes of operation, but still share common optical fiberstretchers.

FIG. 2A shows a schematic of a first associated two optical fiberinterferometer 20A embodiment of the present invention. The firstassociated two optical fiber interferometer 20A incorporates a referenceoptical fiber stretcher 10R having an outer surface with a uniformperimeter called a reference outer surface 12R defining a referenceperimeter and first and second reference delay optical fibers 1R (dashedlines) and 2R (solid lines) respectively being wound around thereference outer surface 12R of the reference optical fiber stretcher 10Ras shown in FIG. 1A. The reference optical fiber stretcher 10R alsocomprises a reference actuator configured to temporally vary thereference perimeter of the reference outer surface 12R of optical fiberstretcher 10R. In the preferred embodiment the reference optical fiberstretcher 10R is comprised of a piezoelectric cylinder which functionsas the reference actuator. Also, the first and second reference delayoptical fibers 1R and 2R are interleaved with each other forming aninterleaved arrangement as they are wound together around the referenceouter surface 12R of the reference optical fiber stretcher 10R and thelengths of the portions of the first and second reference delay opticalfibers 1R and 2R which are interleaved and wound together around thereference outer surface 12R of the reference optical fiber stretcher arethe same. It is also preferred that the first and second reference delayoptical fibers 1R and 2R are wound together around the reference outersurface 12R of the reference optical fiber stretcher 10R in a singlelayer, and that adjacent interleaved delay optical fibers are in contactwith each other when wound around the reference perimeter of thereference optical fiber stretcher 10R.

During operation, the optical path length of light of the samewavelength being transmitted along the first reference delay opticalfiber 1R and the second reference delay optical fiber 2R of thereference optical fiber stretcher 10R are simultaneously varied by thesame amount. The reference optical fiber stretcher 10R is located in thereference branch R of the first associated two optical fiberinterferometer 20A. The input ends of the first reference delay opticalfiber 1R, of the second reference delay optical fiber 2R and of thereference optical fiber stretcher 10R are defined as being on the leftside of the reference optical fiber stretcher 10R; and the output endsof the first reference delay optical fiber 1R, of the second referencedelay optical fiber 2R and of the reference optical fiber stretcher 10Rare defined as being on the right side in FIG. 2A and subsequentfigures.

Coherent light having wavelength λ_(c) from a coherent light source 22-1is coupled into a first fiber optic coupler 36-1 which divides thecoherent light into reference and sample coherent light portions. Thereference and sample coherent light portions are transmitted along twoseparate optical fibers which together make up part of the two branchesof a coherent light Michelson interferometer (parts are indicated withsuffix -1 and dashed lines). The first fiber optic coupler 36-1 ispreferably a 2 by 2 coupler having a 50/50 power splitting ratio so thathalf of the light intensity is transmitted along each branch of thecoherent light Michelson interferometer. Thus, the first fiber opticcoupler 36-1 receives coherent light of wavelength λ_(c) from coherentlight source 22-1 and transmits a first portion of the coherent lightinto an input end of the first reference delay optical fiber 1R andtransmits a second portion of the coherent light into an input end of anoptical fiber in the sample branch of the interferometer. In thisembodiment the optical fiber in the sample branch of the interferometeris called a first sample path matching optical fiber 1S-P which hasabout the same length as reference delay fiber 1R.

In the reference branch of the coherent light Michelson interferometer,the reference coherent light portion is coupled to the first referencedelay optical fiber 1R at the input end of the reference optical fiberstretcher 10R and is then incident onto a first reference reflector48R-1 which is coupled to the output end of the first reference delayoptical fiber 1R of the reference optical fiber stretcher 10R. A portionof the reference coherent light portion is then reflected back fromfirst reference reflector 48R-1 back through the first reference delayoptical fiber 1R of the reference optical fiber stretcher 10R and thenback through the first fiber optic coupler 36-1 and a portion of thisreflected light is incident on a coherent light detector 25-1. Thus, thefirst reference reflector 48R-1 coupled to the output end of the firstreference delay optical fiber 1R reflects coherent light back throughthe first reference delay optical fiber 1R, and back through the firstfiber optic coupler 36-1 into the coherent light detector 25-1.

The sample coherent light portion is coupled to the first sample pathmatching optical fiber 2S-P located in the sample branch S of thecoherent light Michelson interferometer through the first fiber opticcoupler 36-1 and is incident on a first sample reflector 48S-1 as shownin FIG. 2A. A portion of the second coherent light portion reachingfirst sample reflector 48S-1 is reflected back through the first 2 by 2fiber optic coupler 36-1 and a portion of this reflected light and isalso incident on the coherent light detector 25-1. Thus, the firstsample reflector coupled to an output end of the first sample pathmatching optical fiber 2S-P reflects coherent light back through thefirst sample path matching optical fiber 2S-P, and back through thefirst fiber optic coupler 36-1 and into the coherent light detector25-1.

As the optical path of the reference fiber 1R is monotonically changed,a coherent light interference signal will be induced in the coherentlight detector 25-1 as reflected coherent light coming from the sampleand reference branches of the coherent light Michelson interferometerrecombine at the first fiber optic coupler 36-1. The interference signalobserved as a function of time or optical delay distance is called aninterferogram. Since the light is coherent, the interference pattern issinusoidal with a period of λ_(c)/2 where λ_(c) is the wavelength of thecoherent light source 22-1. The coherent light signal incident on thecoherent light detector 25-1 is thus a sinusoidal interference patternhaving a period of λ_(c)/2. Typically the zero crossings of the coherentlight interferometer signal or a subset of them are used as a constantdistance interval distance scale for sampling of the low coherenceinterferometer signal.

Reflectors used in the various embodiments labelled 48R-1-48R-m (in thereference branch) and 48S-1-48S-m (in the sample branch) may be mirrorsor they may be mirrors which also include Faraday rotators. Faradayrotators function to rotate the polarization of the beam to compensatefor the changes in phase of light which occur when light reflects from asurface. The reflectors may also be adjustable in distance so that theycan be moved to a location which matches the path lengths in the twoarms of the Michelson interferometer and then locked into place at anappropriate distance.

The first associated two optical fiber interferometer embodiment shownin FIG. 2A also includes a first low coherence Michelson Interferometerhaving parts indicated with suffix -2 and solid lines. First lowcoherence light from a first low coherence light source 24-2 havingcenter wavelength λ₁ is coupled into an optional first wavelengthdivision multiplexer 34-2 and then into a second fiber optic coupler36-2. (If the optional first wavelength division multiplexer 34-2 werenot present the first low coherence light would be directly coupled intothe second fiber optic coupler 36-2). The other input branch of theoptional first wavelength division multiplexer 34-2 is coupled to anoptional first visible light source 32-2 which is used to identify thelocation being measured (L-2) on an object 42. The second fiber opticcoupler 36-2 divides the first low coherence light into first referenceand first sample low coherence light portions which are transmittedalong two separate optical fibers which together make up part of the twobranches of a first low coherence light Michelson interferometer. The 2by 2 fiber optic coupler 36-2 also transmits a portion of the optionalfirst visible light from the optional visible light source 32-2. Thesecond fiber optic coupler 36-2 is preferably a 2 by 2 fiber opticcoupler having a 50/50 power splitting ratio so that half of the lightintensity is transmitted along each branch of the first low coherencelight Michelson interferometer. Thus, the second fiber optic coupler36-2 receives first low coherence light of center wavelength λ₁ from thefirst low coherence light source 24-2 and transmits a first portion ofthe first low coherence light into an input end of the second referencedelay optical fiber 2R and transmits a second portion of the first lowcoherence light into an input end of the second sample path matchingoptical fiber 2P.

In the reference branch of the first low coherence light Michelsoninterferometer, the first reference low coherence light portion iscoupled to the input end of the second reference delay optical fiber 2Rat the input end of the reference optical fiber stretcher 10R and isthen incident onto a second reference reflector 48R-2 located at theoutput end of the second reference delay optical fiber 2R of thereference optical fiber stretcher 10R. A portion of the first referencelow coherence light portion that is incident on the second referencereflector 48R-2 is then reflected back through the second referencedelay optical fiber 2R of the reference optical fiber stretcher 10R andthen back through the second fiber optic coupler 36-2 and a portion ofthis reflected light is incident on a first low coherence light detector26-2. Thus, the second reference reflector 48R-2 coupled to an outputend of the second reference delay optical fiber 2R reflects first lowcoherence light back through the second reference delay optical fiber2R, and back through the second fiber optic coupler 36-2 into the firstlow coherence light detector 26-2.

In the sample branch S of the first low coherence light Michelsoninterferometer, the first sample low coherence light portion is coupledto the input end of the second sample path matching optical fiber 2S-P.The output end of the second sample path matching optical fiber of thefirst low coherence light Michelson interferometer is coupled to a firstoptical probe 44-2 which focuses or collimates light on the object 42 ata first measurement location L-2 as shown in FIG. 2A. The first opticalprobe 44-2 usually includes lenses in order to focus the low coherencelight onto the first measurement location L-2 on the object 42. Thefirst optical probe 44-2 may optionally collimate the light whichprovides infinite depth of focus (which is still considered focusing).The dotted lines emanating from the output end of optical probe 44-2show the light being focused onto the object 42 at the first measurementlocation L-2. Some visible light from optional first visible lightsource 32-2 will also illuminate the first measurement location L-2 withvisible light. A portion of the sample low coherence light portion fromfirst low coherence light source 24-2 reaching the object 42 at thefirst measurement location (referred to as first location) L-2 isreflected back from one or more of the optical interfaces of the object42 back through the first optical probe 44-2, back through the secondsample path matching optical fiber 2S-P and then back through the secondfiber optic coupler 36-2 and is also incident into the first lowcoherence light detector 26-2. Thus, the first optical probe 44-2coupled to the output end of the second sample path matching opticalfiber 2S-P is configured to transmit first low coherence light to thefirst location L-2 on the object 42 comprising at least one opticalinterface, and to receive first low coherence light reflected back fromthe at least one optical interface, and transmit the reflected first lowcoherence light back through the first optical probe 44-2, back throughthe second sample path matching optical fiber 2S-P, and back through thesecond fiber optic coupler 36-2 and into the first low coherence lightdetector 26-2.

The low coherence light reflected from each of the optical interfaces ofthe object 42 at first measurement location L-2 located in the samplebranch of the first low coherence Michelson interferometer can be madeto interfere with the low coherence light returning along the referencebranch of the first low coherence light Michelson interferometer whichis reflected from the second reference reflector 48R-2. Constructiveinterference will occur when the optical path lengths of the light beingtransmitted along the reference and sample branches of the low coherenceinterferometer are equal and when they differ by optical distancesbetween the various optical interfaces of the device under test. Theconstructive interference at each optical interface will persist for adistance equal to a few coherence lengths of the low coherence lightsource. In order to ensure that constructive interference can occur, thelength of the second sample path matching optical fiber 2S-P must beabout the same length as reference delay fiber 2R.

During operation the location of the second reference reflector 48R-2 inthe reference branch of the low coherence interferometer can bepre-adjusted and set up to a fixed distance in which the optical pathsfrom the second fiber optic coupler 36-2 to the object 42 and thedistance from the second fiber optic coupler 36-2 to the adjustabledistance reflector 48R-2 closely match each other. A periodic referencewaveform (voltage ramp) is applied to the electrodes of the referenceactuator of the reference optical fiber stretcher 10R in order to causethe optical path length of the reference branch of the first lowcoherence light interferometer to cyclically vary from a distance lessthan the optical path length of the sample branch to the first opticalinterface of the object 42 at measurement location L-2 to a distancegreater than the optical path length of the sample branch to the lastoptical interface of the object 42 at measurement location L-2. As thevoltage applied to the reference optical fiber stretcher 10R is varied,constructive interference will occur in the low coherence interferometerwhen the path length of the sample and reference branches of the lowcoherence interferometer are equal and when they differ by an amountequal to distances between optical interfaces in the object 42 atmeasurement location L-2. This interference is observed in the first lowcoherence light detector 26-2 and is analyzed in real time.

As described above with respect to FIG. 1A-FIG. 1C, when the lengths ofthe first and second reference delay optical fibers 1R and 2R that areinterleaved and wound together around the reference outer surface of thereference optical fiber stretcher 10R are the same, the length of thefirst and second reference delay optical fibers 1R and 2R will change bythe same amount as the reference perimeter of the reference opticalfiber stretcher is uniformly varied. In communication grade opticalfibers, the index of refraction of the optical fiber is a function ofthe wavelength of light and dispersion is a function of temperature.

Light of different wavelengths transmitted along optical fibers willalso exhibit differential dispersion effects with temperature. Theassociated two optical fiber interferometer 20A (as compared to using adevice with two single fiber interferometers) offers the advantage thatthe wavelength λ_(c) of the coherent light source 22-1 and the centerwavelength λ₁ of the low coherence light source 24-2 transmitted alongthe first and second reference delay optical fibers 1R and 2R can bemade to be equal to each other. This causes light of the same wavelengthtransmitted along the first and second reference delay optical fibers 1Rand 2R to undergo the same optical path length change as the referenceoptical fiber stretcher 10R changes diameter during operation since thewavelengths are equal, the indices of refraction of the two delayoptical fibers are equal, and thus the optical path length changes areequal. Using coherent and low coherence light sources that have the samecenter wavelength also helps minimize temperature dependent dispersioneffects in the optical fibers.

FIG. 2B shows a schematic of a second associated two optical fiberinterferometer 20B embodiment of the present invention. Only additionalcomponents and changes from the apparatus shown in FIG. 2A will bedescribed in the pursuing discussion. In addition to all of thecomponents in the first associated two optical fiber interferometerembodiment 20A shown in FIG. 2A which all have the same functions, thesecond associated two optical fiber interferometer 20B incorporates asecond optical fiber stretcher called the sample optical fiber stretcher10S. The sample optical fiber stretcher 10S comprises a sample outersurface 12S defining a sample perimeter and two delay optical fibers 1S(dashed lines) and 2S (solid lines) called the first sample delayoptical fiber 1S and the second sample delay optical fiber 2Srespectively wound together around the sample outer surface 12S of thesample optical fiber stretcher 10S in a single layer (as described andshown in FIG. 1A), and the sample optical fiber stretcher 10S is locatedin the sample branch of each of the respective coherent and lowcoherence interferometers. The sample optical fiber stretcher 10Sfurther comprises a sample actuator configured to temporally vary thesample perimeter of the sample outer surface 12S. In the preferredembodiment the sample optical fiber stretcher 10R is comprised of apiezoelectric cylinder having the same perimeter as the referenceactuator. Also in the preferred embodiment, the first and second sampledelay optical fibers 1S and 2S are interleaved with each other as theyare wound together around the sample outer surface 12S of the sampleoptical fiber stretcher 10S forming an interleaved arrangement. Also,the lengths of the portions of the first and second sample delay opticalfibers 1S and 2S which are interleaved and wound together around thereference outer surface of the reference optical fiber stretcher are thesame. It is also preferred that all adjacent interleaved delay opticalfibers are in contact with each other during winding around the sampleperimeter of the sample optical fiber stretcher 10S.

The sample coherent light portion of the coherent light from coherentlight source 22-1 transmitted along the sample branch of the coherentlight Michelson interferometer from the first fiber optic coupler 36-1is coupled to the input end of the first sample delay optical fiber 1Sof the sample optical fiber stretcher 10S and is then incident onto afirst sample reflector 48S-1 located past the output end of sampleoptical fiber stretcher 10S. A portion of the sample coherent lightreaching the first sample reflector 48S-1 is then reflected back throughfirst sample delay optical fiber 1S of the sample optical fiberstretcher 10S and then back through the first fiber optic coupler 36-1and a portion of this reflected light is incident on coherent lightdetector 25-1. Thus, the first sample reflector 48-1 coupled to theoutput end of the first sample delay optical fiber 1S reflects coherentlight back through the first sample delay optical fiber 1S, and backthrough the first fiber optic coupler 36-1 and into the coherent lightdetector 25-1.

Low coherence light from first low coherence light source 24-2transmitted along the sample branch of the first low coherence lightMichelson interferometer transmitted through the second fiber opticcoupler 36-2 called the sample low coherence light portion is coupledinto the second sample delay optical fiber 2S of the sample opticalfiber stretcher 10S and is then incident on the first optical probe 44-2located past the output end of the sample optical fiber stretcher 10S.As described before, the optical probe 44-2 focuses or collimates lighton the object 42 at measurement location L-2. A portion of the lowcoherence light from first low coherence light source 24-2 reaching theobject 42 is reflected back from each of the optical interfaces of theobject 42 at measurement location L-2 back through the first opticalprobe 44-2, the second sample delay optical fiber 2S of the sampleoptical fiber stretcher 10S, and the second fiber optic coupler 36-2,and is incident onto the first low coherence light detector 26-2 asdescribed above with respect to the first two fiber interferometer 20Ashown in FIG. 2A. Thus, the second fiber optic coupler 36-2 receivesfirst low coherence light of center wavelength λ₁ from the first lowcoherence light source 24-2 and transmits a first portion of the firstlow coherence light into the input end of the second reference delayoptical fiber 2R and transmits a second portion of the first lowcoherence light into the input end of the second sample delay opticalfiber 2S. The first optical probe 44-2 coupled to the output end of thesecond sample delay optical fiber 2S is configured to transmit first lowcoherence light to a first location L-2 on an object 42 comprising atleast one optical interface, and receive first low coherence lightreflected back from the at least one optical interface, and transmit thereflected first low coherence light back through the first optical probe44-2, back through the second sample delay optical fiber 2S, and backthrough the second fiber optic coupler 36-2 and into the first lowcoherence light detector 26-2.

During operation of the second associated two optical fiberinterferometer 20B shown in FIG. 2B, the pair of reference and sampleoptical fiber stretchers 10R and 10S respectively are preferablyoperated in a push-pull mode. When operating in the push-pull mode, thereference actuator and the sample actuator are driven with respectivereference and sample waveforms that monotonically increase the referenceperimeter the reference optical fiber stretcher while monotonicallydecreasing the sample perimeter of the sample optical fiber stretcherrespectively and vice versa. Thus, the fiber optic path lengths of thefirst and second reference delay optical fibers are increasing while thefiber optic path lengths of the first and second sample delay opticalfibers are decreasing and vice versa. In the case of a piezoelectricactuator, the waveform is the voltage applied to the piezoelectric coilactuator as a function of time and the waveform is preferred to beperiodic and continuous. When operating in the push-pull mode thevoltage waveforms applied to the reference optical fiber stretcher 10Rand the sample optical fiber stretcher 10S are ramped in oppositedirections as a function of time as shown in FIG. 9. This results in thesecond two fiber interferometer 20B having double the maximum fiberoptic path length change that can be obtained in the first 2 fiberinterferometer 20A shown in FIG. 2A. The preferred voltage waveformapplied to the piezoelectric cylinders is a triangle wave as shown inFIG. 9. In the preferred embodiment, the lengths of the first referencedelay optical fiber 1R, the second reference delay optical fiber 2R, thefirst sample delay optical fiber 1S, and the second sample delay opticalfiber 2S wound around the individual two optical fiber stretchers areall the same.

The reference and sample optical fiber stretchers 10R and 10S arepiezoelectric cylinders having the same diameter in the preferredembodiment. FIG. 9 shows an actuator voltage versus time graph 90showing the reference and sample waveforms applied to the reference andsample piezoelectric actuators as a function of time for the push-pullmode of operation. The reference and sample waveforms are periodic withrespect to time and are shown as the voltages applied to thepiezoelectric cylinder actuators in volts as a function of time. FIG. 9also shows the change in optical path difference between the sample andreference branches of the associated optical fiber interferometers in mmversus time for a pair of reference and sample piezoelectric cylinderactuators of a Michelson interferometer operating in the push-pull mode.The dashed line waveform (- - -) shows the voltage applied to thereference actuator of the reference optical fiber stretcher 10R on thevertical axis versus time on the horizontal axis. The solid linewaveform (——) shows the voltage applied to the sample actuator of thesample optical fiber stretcher 10S on the vertical axis also versus timeon the horizontal axis. The dotted line curve (⋅ ⋅ ⋅ ⋅ ⋅) labeled Δ OPDshows the difference in the optical path length between the sample andreference branches of the coupled interferometers as a function of time.The optical path difference between the reference and sample branchesare also periodic in time with the same period (20 relative time unitsin FIG. 9) as the periodic drive voltages on the individualpiezoelectric coils. During operation, the coherent light interferometerand the first low coherence light interferometer both exhibit the sameoptical path difference as a function of time.

As in the case of the first associated two optical fiber interferometer20A, the low coherence light reflected from each of the opticalinterfaces of the object 42 located in the sample branch of the firstlow coherence Michelson interferometer of the second 2 fiberinterferometer 20B can be made to interfere with the low coherence lightreturning along the reference branch of the first low coherence lightMichelson interferometer, which is reflected back from referencereflector 48R-2. As described before constructive interference willoccur when the optical path lengths of the light being transmitted alongthe two branches of the low coherence interferometer are equal and whenthey differ by an amount equal to distances between optical interfacesin the object 42 at measurement location L-2.

Using a pair of optical fiber stretchers having the same length ofoptical fiber in both branches of the associated interferometers makesit easier to match path lengths in the two branches of each of theassociated interferometers. As in the case of the first associated twooptical fiber interferometer 20A, the location of the referencereflector 48R-2 in the reference branch of the first low coherenceinterferometer of the second associated two optical fiber interferometer20B is pre-adjusted and set to a fixed distance in which the opticalpaths from the second 2 by 2 fiber optic coupler 36-2 to the object 42and the distance from the second 2 by 2 fiber optic coupler 36-2 to thereference reflector 48R-2 closely match each other. In discussions ofall subsequent interferometer configurations of the present invention,the path lengths of the reference and sample branches of each lowcoherence interferometer are matched so that interference will occurover the scan distance of the optical fiber stretchers.

FIG. 2C shows a schematic of a third associated two optical fiberinterferometer 20C embodiment of the present invention. Most of thecomponents in the third associated two optical fiber interferometer 20Care the same as in the second associated two optical fiberinterferometer embodiment 20B shown in FIG. 2B and all of the samecomponents have the same functions. In the third associated two opticalfiber interferometer 20C embodiment the first low coherence lightdetector 26-2 is a first balanced detector 28-2. In a balanced detector,the detector includes a second input so that the two input signals canbe subtracted from each in order to remove common mode noise between thetwo inputs of the balanced detector. A first optical circulator which werefer to as circulator 38-2 is also placed between the first lowcoherence light source 24-2 and the second fiber optic coupler 36-2 andits operation is described below.

Circulators are three port devices that function as optical isolatorswhich allow light to propagate in one direction being from the firstport to the second port of the circulator and from the second port tothe third port of the circulator, but not in the reverse direction. Theclockwise arrow inside first circulator 38-2 indicates the direction inwhich light propagates inside the circulator. In FIG. 2C, the first orinput port of circulator 38-2 is coupled to the first low coherencelight source 24-2, the second port is coupled to the optional firstwavelength division multiplexer 34-2 and the third port is coupled toone of the inputs of the balanced detector 28-2. (In the schematics ofFIGS. 2C and 3B-6B, circulators are represented by open circles and thethree ports are indicated by small solid circles overlapping the opencircle. The curved arrow shown inside the circulator indicates thedirection of light propagation and the back end of the arrow is near thefirst port of the circulator.)

In the third associated two optical fiber interferometer 20C embodimentof the present invention the first low coherence light detector 26-2 ofinterferometers 20A (FIG. 2A) and 20B (FIG. 2B) is a first balanceddetector 28-2, and a first circulator 38-2 is placed between the firstlow coherence light source 24-2 and the optional first wavelengthdivision multiplexer 34-2. When the optional first wavelength divisionmultiplexer 34-2 is not present, the second port of the first circulator38-2 is coupled into the input end of the second fiber optic coupler36-2.

The second fiber optic coupler 36-2 receives reflected light coming backfrom the second reference reflector 48R-2 that interferes with the lowcoherence light reflected from each of the optical interfaces of theobject 42 at measurement location L-2. A portion of this interferingreflected light is also incident on port 2 of the first circulator 38-2.This interfering reflected light is then transmitted to port 3 of thefirst circulator 38-2 and is incident on the first input of balanceddetector 38-2. A second portion of the interfering reflected light isinput directly to the second input of balanced detector 28-2. Theassociated two optical fiber interferometer 20A shown in FIG. 2A mayalso be modified to include the first circulator 38-2 and the firstbalanced detector 28-2 instead of detector 26-2. Thus in the thirdassociated two optical fiber interferometer 20C, the first low coherencelight detector is a first balanced detector 28-2, and the interferometerapparatus also comprises a first circulator 38-2 comprised of a firstport coupled to the first low coherence light source 24-2, a second portcoupled to the second fiber optic coupler 36-2, and a third port coupledto the first input port of the first balanced detector 28-2. The secondinput port of the first balanced detector is configured to receive thefirst low coherence light reflected back from the at least one opticalinterface at the first location L-2 on the object 42 combined with thelight reflected back from the second reference reflector 48R-2 andpassing back through the second fiber optic coupler 36-2.

FIG. 3A shows a schematic of a first associated three optical fiberinterferometer 30A embodiment of the present invention. Most of thecomponents in the first associated three optical fiber interferometer20C are the same as in the second associated two optical fiberinterferometer embodiment 20B shown in FIG. 2B and all of the commoncomponents have the same functions. Comparing the embodiment 20B shownin FIG. 2B to the embodiment 30A shown in FIG. 3A, the reference opticalfiber stretcher 13R further comprises a third reference delay opticalfiber 3R wound around the reference outer surface 12R, and the sampleoptical fiber stretcher 13S further comprises a third sample delayoptical fiber 3S wound around the sample outer surface. The pair ofoptical fiber stretchers have outer surfaces having uniform perimeters(a reference optical fiber stretcher 13R and a sample optical fiberstretcher 13S), with the three fibers each being 1R (dashed lines), 2R(solid lines) and 3R (dotted lines) and 1S (dashed lines), 2S (solidlines) and 3S (dotted lines) respectively wound around them in the twobranches of the coherent light and low coherence light Michelsoninterferometers. The first, second and third reference delay opticalfibers 1R, 2R and 3R of the reference optical fiber stretcher 13R arewound together around the reference outer surface of the referenceoptical fiber stretcher preferably in a single layer. The first, secondand third sample delay optical fibers 1S, 2S, and 3S of the sampleoptical fiber stretcher 13S are also wound together around the sampleouter surface of the sample optical fiber stretcher 13S preferably in asingle layer. The reference optical fiber stretcher 13R and the sampleoptical fiber stretcher 13S further comprise reference and sampleactuators for temporally varying the reference and sample perimeters ofthe reference and sample optical fiber stretchers 13R and 13Srespectively.

In a preferred embodiment the reference and sample optical fiberstretchers 13R and 13S are comprised of piezoelectric cylinders havingthe same perimeter which function as the respective reference and sampleactuators. Also in this embodiment the first, second, and thirdreference delay optical fibers 1R, 2R, and 3R are preferably interleavedwith each other as they are wound together around the reference outersurface 12R of the reference optical fiber stretcher 13R forming aninterleaved arrangement and the first, second, and third sample delayoptical fibers 1S, 2S, and 3S are interleaved with each other as theyare wound together around the sample outer surface 12S of the sampleoptical fiber stretcher 13S forming an interleaved arrangement. Also thelengths of the first, second, and third reference delay optical fibers1R, 2R, and 3R which are interleaved and wound together around thereference outer surface 12R of the reference optical fiber stretcher 13Rand the lengths of the first, second and third sample delay opticalfibers 1S, 2S and 3S being interleaved and wound together around thesample outer surface 12S of the sample optical fiber stretcher 13S arepreferably the same. It is also preferred that all adjacent interleavedreference delay optical fibers 1R, 2R, and 3R and all adjacentinterleaved sample delay optical fibers 1S, 2S, and 3S are in contactwith each other during winding around the reference and sampleperimeters of the reference and sample optical fiber stretchers 13R and13S respectively.

The first associated three optical fiber interferometer 30A alsoincludes a second low coherence Michelson interferometer (parts areindicated with suffix -3 and dotted lines), which is coupled to thethird reference delay optical fiber 3R and the third sample delayoptical fiber 3S which are wound around the reference optical fiberstretcher 13R and the sample optical fiber stretcher 13S which operatein the push-pull mode as described above with reference to thediscussion of FIG. 2B. Second low coherence light from a second lowcoherence light source 24-3 having wavelength λ₂ is coupled into anoptional second wavelength division multiplexer 34-3 and then into athird fiber optic coupler 36-3. (If the optional second wavelengthdivision multiplexer 34-3 were not present, the first low coherencelight would be directly coupled into the third fiber optic coupler36-3). The other input arm of the optional second wavelength divisionmultiplexer 34-3 is coupled to an optional second visible light source32-3 which is used to identify the location being measured L-3 on theobject 42.

The third fiber optic coupler 36-3 splits the low coherence light fromthe second low coherence light source 24-3 into second reference andsecond sample low coherence light portions which propagate through thetwo branches of a second low coherence light Michelson interferometer.The second reference and second sample low coherence light portions arecoupled to the input ends of the third reference delay optical fiber 3Rand the third sample delay optical fiber 3S, which are wound around theoptical fiber stretchers 13R and 13S respectively. Thus, the third fiberoptic coupler 36-3 receives second low coherence light of centerwavelength λ₂ from a second low coherence light source 24-3 andtransmits a first portion of the second low coherence light into aninput end of the third reference delay optical fiber 3R and transmits asecond portion of the second low coherence light into an input end ofthe third sample delay optical fiber 3S. The third fiber optic coupler36-3 may have any splitting ratio but it is preferably a 50/50 couplerso that half of the light intensity is transmitted along each branch ofthe first low coherence light Michelson interferometer. In the referencebranch of the second low coherence light Michelson interferometer, aportion of the low coherence light from the second low coherence lightsource 24-3 passing through the third fiber optic coupler 36-3 iscoupled to the third reference fiber 3R of the three optical fiberstretcher 13R and is then incident onto a third reference reflector48R-3. The location of the third reference reflector 48R-3 ispre-adjusted and set up to a fixed distance in the same way as thesecond reference reflector 48R-2 as described above in the discussion ofFIG. 2A. A portion of the second low coherence light from second lowcoherence light source 24-3 that reaches the third reference reflector48R-3 is reflected back through the third reference delay optical fiber3R of the reference optical fiber stretcher 13R and then back throughthe third fiber optic coupler 36-3, and a portion of the second lowcoherence reflected light is incident on a second low coherence lightdetector 26-3. Thus, the third reference reflector coupled to the outputend of the third reference delay optical fiber 3R reflects second lowcoherence light back through the third reference optical fiber 3R, andback through the third fiber optic coupler 36-3 and into a second lowcoherence light detector 26-3.

A portion of the second low coherence light from the second lowcoherence light source 24-3 transmitted along the sample branch of thesecond low coherence light Michelson interferometer from the third fibercoupler 36-3 is coupled into the third sample delay optical fiber 3S ofthe sample optical fiber stretcher 13S, and is then incident on a secondoptical probe 44-3 which focuses or collimates light on the object 42having at least one optical interface at measurement location L-3 asshown in FIG. 3A. A portion of the low coherence light from the secondlow coherence light source 24-3 reaching the object 42 at measurementlocation L-3 is reflected back from each of the optical interfaces ofthe object 42 back through the second optical probe 44-3, back throughthe third sample delay optical fiber 3S of the sample optical fiberstretcher 10S and back through the third fiber optic coupler 36-3 and isalso incident onto the second low coherence light detector 26-3. Thus,the second optical probe 44-3 coupled to the output end of the thirdsample delay optical fiber 3S is configured to transmit second lowcoherence light to a second location L-3 on the object 42 and receivesecond low coherence light reflected back from the at least one opticalinterface, and transmit the reflected second low coherence light backthrough the second optical probe 44-3, back through the third sampledelay optical fiber 3S and back through the third fiber optic coupler36-3 and into the second low coherence light detector 26-3. As in thecase of the second low coherence interferometer shown in FIG. 2B,constructive interference will occur between the reference R and sampleS branches of the second low coherence interferometer when the pathlengths of the reference and sample branches are the same and when theydiffer by an amount equal to distances between optical interfaces in theobject 42 at the measurement location now at L-3.

FIG. 3B shows a schematic of a second associated three optical fiberinterferometer 30B embodiment of the present invention. Most of thecomponents in the second associated three optical fiber interferometer30B are the same as in the first associated three optical fiberinterferometer embodiment 30A shown in FIG. 3A and all of the samecomponents have the same functions. As in the third associated twooptical fiber interferometer 20C, the first low coherence light detector26-2 is a first balanced detector 28-2, and a first circulator 38-2 isplaced between the first low coherence light source 24-2 and theoptional first wavelength division multiplexer 34-2. The first port ofthe first circulator 38-2 is coupled to the first low coherence lightsource 24-2, the second port is coupled to the optional first wavelengthdivision multiplexer 34-2, and the third port is coupled to one of theinputs of the first balanced detector 28-2. The other input of thebalanced detector 28-2 is coupled to the second 2 by 2 fiber opticcoupler 36-2, which receives a portion of the reflected light comingfrom the second reference reflector 48R-2 that interferes with the lowcoherence light reflected from each of the optical interfaces of theobject 42 at measurement location L-2.

Also in the second three fiber interferometer 30B, the second lowcoherence light detector 26-3 is a second balanced detector 28-3, and asecond circulator 38-3 is placed between the second low coherence lightsource 24-3 and the second optional wavelength division multiplexer34-3. The first or input port of circulator 38-3 is coupled to thesecond low coherence light source 24-3, the second port is coupled tothe second optional wavelength division multiplexer 34-3 and the thirdport is coupled to one of the inputs of the second balanced detector28-3. When the optional second wavelength division multiplexer 34-3 isnot present, the second port of the second circulator 38-3 is coupledinto the input end of third fiber optic coupler 36-3. The third fiberoptic coupler 36-3, receives reflected light coming back from the thirdreference reflector 48R-3 that interferes with the second low coherencelight portion being reflected from each of the optical interfaces of theobject 42 at measurement location L-3. A portion of this interferingreflected light is also incident on port 2 of the second circulator38-3. This interfering reflected light is then transmitted to port 3 ofthe second circulator 38-3 and is incident on the first input of thesecond balanced detector 28-3. A second portion of the interferingreflected light is input directly to the second input of the secondbalanced detector 28-3. In summary, in the second associated threeoptical fiber interferometer 30B embodiment of the present invention,the detector is a second balanced detector 28-3. The interferometerapparatus 30B further comprises a second circulator 38-3 comprised of afirst port coupled to the second low coherence light source 24-3, asecond port coupled to the third fiber optic coupler 36-3, and a thirdport coupled to a first input port of the second balanced detector 28-3,a second input port of the second balanced detector 28-3 configured toreceive the second low coherence light reflected back from the at leastone optical interface at the second location L-3 on the object 42combined with the light reflected back from the third referencereflector 48R-3 and passing back through the third fiber optic coupler36-3.

Although the associated three optical fiber interferometerconfigurations shown in FIGS. 3A-3B show configurations having a matchedpair of reference and sample optical fiber stretchers 13R and 13Srespectively having three delay optical fibers each in the reference andsample branches of each of the three associated interferometers, it ispossible to build an associated three optical fiber interferometerhaving a single common optical fiber stretcher containing three delayoptical fibers. In such a configuration, the single common optical fiberstretcher simultaneously varies the optical path length of lighttransmitted along the three optical fibers by the same amount. In thiscase, the three optical fibers would be located only in the referencebranch of each of the three associated optical fiber interferometerssimilar to the configuration shown in FIG. 2A.

In preferred embodiments of the associated three optical fiberinterferometers shown in FIG. 3A-3B, λ_(c), λ₁, and λ₂ are equal. Thefirst and second low coherence light sources 26-2 and 26-3 may be madefrom the same light source and may share a common emitter. As anexample, they may be fiber coupled and split into two light sources withan optical fiber coupler.

FIG. 4A-FIG. 6B show schematics of various associated autocorrelatorembodiments of the present invention. In all of these embodiments, thecoherent light interferometer is in a standard Michelson configurationand has the identical components to those described in FIG. 2B-FIG. 3C.In all of these embodiments, at least one of the associated lowcoherence light interferometers is configured as an autocorrelator wherethe sample is at the input to the Michelson interferometer.

FIG. 4A shows a first associated two optical fiber autocorrelator 40Aembodiment of the present invention. First low coherence light fromlight from first low coherence light source 24-2 is coupled into thefirst port of a first primary circulator 38 b-2, which sends lightthrough an optional first wavelength division multiplexer 34-2 andthrough the first optical probe 44-2, which focuses or collimates lighton the object 42 at measurement location L-2. (We use the primarydesignation on a circulator to indicate that light from a light sourceis coupled to the object before being split into two paths.) The otherinput branch of the first optional wavelength division multiplexer 34-2is coupled to an optional first visible light source 32-2 which is usedto illuminate the location being measured (L-2) on the object 42 withvisible light. A portion of the first low coherence light from first lowcoherence light source 24-2 reaching the object 42 at measurementlocation L-2 is reflected back from one or more of the opticalinterfaces of the object 42 back through the first optical probe 44-2and then back through port 2 of the first primary circulator 38 b-2 andthen through port 3 of the first primary circulator 38 b-2 and is inputinto the second fiber optic coupler 36-2.

A portion of the first low coherence light that is reflected off of theone or more optical interfaces of the object 42 at measurement locationL-2 is then split into reference and sample branches of a first lowcoherence Michelson interferometer at the output end of second fiberoptic coupler 36-2 (parts indicated with suffix -2). The low coherencelight transmitted along the reference and sample branches of the firstlow coherence Michelson interferometer is coupled into second referencedelay optical fiber 2R of reference optical fiber stretcher 10R andsecond sample delay optical fiber 2S of sample optical fiber stretcher10S respectively, and is then incident on second reference reflector48R-2 and second sample reference reflector 48S-2 respectively. A firstreference portion of the first low coherence light that was reflectedoff of the one or more optical interfaces of the object 42 atmeasurement location L-2, which are coupled into the reference branch ofthe interferometer, is reflected back from the second referencereflector 48R-2, back through the second reference delay optical fiber2R of the reference optical fiber stretcher 10R and back through thesecond fiber optic coupler 36-2 and into the first low coherence lightdetector 26-2. Similarly, a first sample portion of the first lowcoherence light that was reflected off of the one or more opticalinterfaces of the object 42 at measurement location L-2, which arecoupled into the sample branch of the interferometer, is reflected backfrom the second sample reflector 48S-2, back through the second sampledelay optical fiber 2S of the sample optical fiber stretcher 10S andback through the second fiber optic coupler 36-2 and into the first lowcoherence light detector 26-2. The first reference portion and the firstsample portion of the reflected low coherence light are recombined atthe fiber optic coupler 36-2 where they interfere with each other. Theinterfering light is then incident on the first low coherence lightdetector 26-2 where the signal is amplified and processed.

As described with reference to the second associated two optical fiberinterferometer 20B shown in FIG. 2B, the reference and sample opticalfiber stretchers 10R and 10S in the first associated two optical fiberautocorrelator 40A shown in FIG. 4A are operated in the push-pull mode.Constructive interference will occur in first low coherence lightdetector 26-2 when the optical path lengths of the reference and samplebranches of the interferometer are equal and when they differ by thedistance between different optical interfaces in the object 42 at themeasurement location L-2.

FIG. 4B shows a schematic of a second associated two optical fiberautocorrelator 40B embodiment of the present invention. Most of thecomponents in the second associated two optical fiber autocorrelator 40Bare the same as in the first associated two optical fiber autocorrelatorembodiment 40A shown in FIG. 4A, and all of the same components have thesame functions. The first low coherence light detector 26-2 is a firstbalanced detector 28-2, and a first circulator 38-2 is placed betweenthe first primary circulator 38 b-2 and the second fiber optic coupler36-2. The first port of the first circulator 38-2 is coupled to thethird port of the first primary circulator 38 b-2 which carries firstlow coherence light that is reflected off the of one or more opticalinterfaces of the object at location L-2, the second port of firstcirculator 38-2 is coupled to the input port of second fiber opticcoupler 36-2, and the third port is coupled to one of the inputs of thefirst balanced detector 28-2. The other input of the first balanceddetector 28-2 is coupled to the second fiber optic coupler 36-2, whichreceives combined interfering light being reflected from secondreference reflector 48R-2 and second sample reflector 48S-2 asdescribed, which reference to the discussion of the configuration shownin FIG. 4A.

FIG. 5A shows a schematic of a first associated three optical fiberautocorrelator 50A embodiment of the present invention. Most of thecomponents in the first associated three optical fiber autocorrelator50A are the same as in the first associated two optical fiberautocorrelator embodiment 40A shown in FIG. 4A and all of the commoncomponents have the same functions. In the first associated threeoptical fiber autocorrelator 50A of FIG. 5A, a pair of optical fiberstretchers, a reference optical fiber stretcher 13R and a sample opticalfiber stretcher 13S, having three fibers each, 1R (dashed lines), 2R(solid lines) and 3R (dotted lines) and 1S (dashed lines), 2S (solidlines) and 3S (dotted lines) respectively, which are interleaved andwound together around them as described with reference to the discussionof FIG. 3A above, replace the reference and sample fiber stretchers 10Rand 10S in the two branches of the coherent light and low coherencelight interferometers shown in FIG. 4A. The first associated threeoptical fiber autocorrelator 50A also includes a second low coherencelight autocorrelator (parts are indicated with suffix -3 and dottedlines), which is coupled to the third reference delay optical fiber 3Rand the third sample delay optical fiber 3S that are wound around thereference optical fiber stretcher 13R and the sample optical fiberstretcher 13S, which operate in the push-pull mode as described abovewith reference to the discussion of FIG. 3A.

The first associated three optical fiber autocorrelator 50A alsoincludes a second low coherence light source 24-3. Low coherence lightfrom the second low coherence light source 24-3 is coupled into thefirst port of a second primary circulator 38 b-3, which sends lightthrough an optional second wavelength division multiplexer 34-3 andthrough a second optical probe 44-3, which focuses or collimates lighton the object 42 at measurement location L-3. The other input branch ofthe optional second wavelength division multiplexer 34-3 is coupled toan optional second visible light source 32-3, which is used toilluminate the location being measured (L-3) on the object 42 withvisible light. A portion of the second low coherence light from secondlow coherence light source 24-3 reaching the object 42 at measurementlocation L-3 is reflected back from one or more of the opticalinterfaces of the object 42 at measurement location L-3 back through theoptical probe 44-3, and then back through port 2 of the second primarycirculator 38 b-3, and then through port 3 of the second primarycirculator 38 b-3, and is input into the third fiber optic coupler 36-3.A portion of the second low coherence light that is reflected off of theone or more optical interfaces of the object 42 at measurement locationL-3 is then split into reference and sample branches of a second lowcoherence Michelson interferometer and coupled into the third referencedelay optical fiber 3R of the reference optical fiber stretcher 13R andthe third sample delay optical fiber 3S of the sample optical fiberstretcher 13S respectively, and is then incident on third referencereflector 48-3 and third sample reflector 48S-3 respectively. Areference portion of the second low coherence light that is reflectedoff of the one or more optical interfaces of the object 42 atmeasurement location L-3 is coupled into the reference branch of thesecond low coherence interferometer and is reflected back from the thirdreference reflector 48R-3, back through the third reference delayoptical fiber 3R of the reference optical fiber stretcher 13R, and backthrough the third fiber optic coupler 36-3, and into the second lowcoherence light detector 26-3. Similarly, a sample portion of the secondlow coherence light that is reflected off of the one or more opticalinterfaces of the object 42 at measurement location L-3 is coupled intothe sample branch of the second low coherence interferometer, and isreflected back from the third sample reflector 48S-3, back through thethird sample delay optical fiber 3S of the sample optical fiberstretcher 13S, and back through the third fiber optic coupler 36-3 andinto the second low coherence light detector 26-3. The reference portionand the sample portion of the reflected second low coherence light arerecombined at the third fiber optic coupler 36-3 where they interferewith each other. This interfering light is then incident on the secondlow coherence light detector 26-3 where the signal is amplified andprocessed. As described with reference to the first associated threeoptical fiber interferometer 30A shown in FIG. 3A, the pair of opticalfiber stretchers, (reference optical fiber stretcher 13R and the sampleoptical fiber stretcher 13S having three optical fibers each) areoperated in the push-pull mode in the first three fiber autocorrelator50A shown in FIG. 5A. Constructive interference will occur at the secondlow coherence light detector 26-3 when the optical path lengths of thetwo arms of the interferometer are equal and when they differ by thedistance between different optical interfaces in the object 42 at themeasurement location L-3.

FIG. 5B shows a schematic of a second associated three optical fiberautocorrelator 50B embodiment of the present invention. Most of thecomponents in the second associated three optical fiber autocorrelator50B are the same as in the first associated three optical fiberautocorrelator embodiment 50A shown in FIG. 5A, and all of the samecomponents have the same functions. As in the second associated twooptical fiber autocorrelator 40B, the first low coherence light detector26-2 is a first balanced detector 28-2, and a first circulator 38-2 isplaced between the first primary circulator 38 b-2 and the second fiberoptic coupler 36-2. The first port of the first circulator 38-2 iscoupled to the third port of the first primary circulator 38 b-2, whichcarries first low coherence light that is reflected off of one or moreoptical interfaces of the object at location L-2, the second port ofcirculator 38-2 is coupled to the input port of the second fiber opticcoupler 36-2, and the third port of circulator 38-2 is coupled to one ofthe inputs of the first balanced detector 28-2. The other input of thefirst balanced detector 28-2 is coupled to the second fiber opticcoupler 36-2, which receives combined interfering light being reflectedfrom the second reference reflector 48R-2 and the second samplereflector 48S-2 as described which reference to the configuration shownin FIG. 5A. Also in the second three fiber autocorrelator 50B, thesecond low coherence light detector 26-3 is a second balanced detector28-3, and a second circulator 38-3 is placed between the second primarycirculator 38 b-3 and the third fiber optic coupler 36-3. The first portof the circulator 38-3 is coupled to the third port of the secondprimary circulator 38 b-3, which carries low coherence light that isreflected off of one or more optical interfaces of the object atlocation L-3, the second port of the second circulator 38-2 is coupledto the input port of the third fiber optic coupler 36-3, and the thirdport of the second circulator 38-3 is coupled to one of the inputs ofthe second balanced detector 28-3. The other input of the secondbalanced detector 28-3 is coupled to the third fiber optic coupler 36-3,which receives combined interfering light being reflected from the thirdreference reflector 48R-3 and the third sample reflector 48S-3, asdescribed with reference to the configuration shown in FIG. 5A.

FIG. 6A and FIG. 6B show schematics of first and second associated threeoptical fiber hybrid interferometers 60A and 60B which combine acoherent standard mode Michelson interferometer with a standard mode lowcoherence Michelson interferometer and a low coherence autocorrelatorinterferometer using a pair of optical fiber stretchers having threefibers each, and being the reference optical fiber stretcher 13R and thesample optical fiber stretcher 13S. The first associated three opticalfiber hybrid interferometer 60A includes a standard mode coherent lightinterferometer having parts labelled with suffix -1 with optical fibershaving dashed lines, and a standard mode low coherence lightinterferometer having parts labelled with suffix -3 with optical fibershaving dotted lines, which have the same components and function as thestandard mode interferometers having suffixes -1 and -3 respectivelyshown in FIG. 3A. The first associated three optical fiber hybridinterferometer 60A also includes a low coherence light interferometer inthe autocorrelator configuration having parts labeled with suffix -2with optical fibers having solid lines, which has the same componentsand function as the low coherence light autocorrelator having suffix -2shown in FIG. 5A. The second associated three optical fiber hybridinterferometer 60B also includes a standard mode coherent lightinterferometer having parts labelled with suffix -1 with optical fibershaving dashed lines, and a standard mode low coherence lightinterferometer labelled with suffix -3 with optical fibers having dottedlines, which have the same components and function as the standard modelow coherence interferometer having suffix -2 shown in FIG. 3B. Thesecond associated three optical fiber hybrid interferometer 60B alsoincludes a low coherence light interferometer in the autocorrelatorconfiguration labeled with suffix -2, which have the same components andfunction as the low coherence light autocorrelator having suffix -2shown in FIG. 5B.

The hybrid interferometer offers the advantage that optical probescoupled to the autocorrelator probe do not have to be matched in pathlength to the path length of a reference arm. It is also to beunderstood that the autocorrelator in the hybrid interferometers shownin FIG. 6A and FIG. 6B may be located in any one of the three fiberinterferometers with suffixes 1-3.

In FIG. 2A-FIG. 3B, the optional first wavelength division multiplexer34-2 for combining optional first visible light from the optional firstvisible light source 32-2 with first low coherence light from lowcoherence light source 24-2 is shown as being on the input side of thesecond fiber optic coupler 36-2. Since the purpose of the first visiblelight source 32-2 is to provide a visible indicator on the object 42 atthe measurement location L-2, the optional first wavelength divisionmultiplexer 34-2 together with the first visible light source 32-2 beingdirectly coupled to it may be instead placed at many other locations inthe various multiple interferometer configurations. The other locationsinclude: 1) between the second 2 by 2 fiber optic couplers 36-2 andfirst optical probe 44-2 (in FIG. 2A); 2) between the second 2 by 2fiber optic couplers 36-2 and the sample optical fiber stretcher 10S or13S (in FIG. 2B-FIG. 3B); or 3) between the sampled optical fiberstretcher 10S or 13S (in FIG. 2B-FIG. 3B) and the first optical probe44-2. The same arguments can be made for the optional second wavelengthdivision multiplexer 34-3 together with the second visible light source32-2 as they may also be located at different locations along the secondlow coherence interferometer. In all cases, the purpose of the secondvisible light source 32-3 is to provide a visible indicator on theobject 42 of the measurement location L-3.

Although FIG. 2A-FIG. 6B describe embodiments showing two and threeassociated interferometers that share either a common reference opticalfiber stretcher or both common reference and common sample optical fiberstretchers which simultaneously vary the optical path length of lighttransmitted along the 2 or 3 optical fibers located in the referencebranch or the reference branch and the sample branch of each of the twoor three associated interferometers by the same amount, it is to beunderstood that there could be m associated optical fiberinterferometers where m is an integer greater than 1. We call thisconfiguration an associated m optical fiber interferometer apparatus.

FIG. 7A shows a first associated m optical fiber interferometerembodiment 70A of the present invention containing m associated standardmode fiber Michelson interferometers which include common reference andcommon sample optical fiber stretchers 16R and 16S respectively having moptical fibers each and reference and sample outer surfaces 12R and 12Swith uniform reference and sample perimeters located in the referenceand sample branches of the associated m optical fiber interferometer 70Arespectively. The m reference delay optical fibers (1R . . . mR) in thereference optical fiber stretcher 16R and the m sample delay opticalfibers (1S . . . mS) in the sample optical fiber stretcher 16S are woundtogether around their respective reference and sample outer surface 12Rand 12S of their respective m reference and sample optical fiberstretchers 16R and 16S preferably in a single layer. The referenceoptical fiber stretcher 16R and the sample optical fiber stretcher 16Sfurther comprise reference and sample actuators for temporally varyingthe reference and sample perimeters of the reference and sample opticalfiber stretchers 16R and 16S respectively. During operation, the commonreference optical fiber stretcher 16R varies the optical path length oflight of the same wavelength transmitted along the m reference delayoptical fibers by the same amount and the common sample optical fiberstretcher 16S varies the optical path length of light of the samewavelength transmitted along the m sample delay optical fibers by thesame amount. In a preferred embodiment, the reference and sample opticalfiber stretchers 16R and 16S are comprised of piezoelectric cylindershaving the same perimeter which function as the respective reference andsample actuators. Also in this embodiment, the m reference delay opticalfibers (1R . . . mR) are interleaved with each other as they are woundtogether around the reference outer surface of the reference opticalfiber stretcher 16R in a single layer, and the m sample delay opticalfibers (1S . . . mS) are interleaved with each other as they are woundtogether around the sample outer surface of the sample optical fiberstretcher 16S in a single layer. Also the lengths of all of the mreference delay optical fibers (1R . . . mR) which are interleaved andwound together around the reference outer surface of the referenceoptical fiber stretcher 16R and the lengths of the all of the m sampledelay optical fibers (1S . . . mS) which are interleaved and woundtogether around the sample outer surface of the sample optical fiberstretcher 16S are preferably the same. It is also preferred that alladjacent interleaved reference delay optical fibers (1R . . . mR) andall adjacent interleaved sample delay optical fibers (1S . . . mS) arein contact with each other during winding around the reference andsample perimeters of the reference and delay optical fiber stretchers16R and 16S respectively. It is also preferred that the referenceoptical fiber stretcher 16R and the sample optical fiber stretcher 16Soperate in the push-pull mode as described above with respect to thediscussion of FIG. 9.

The first associated m optical fiber interferometer embodiment 70A ofthe present invention shown in FIG. 7A shows only the components in the1^(st) and m^(th) interferometers shown having suffixes -1 and -m anddelay optical fibers 1R, 1S, mR and mS. Each of the m associatedinterferometers also includes a light source (24-1 . . . 24-m) whichprovides light to a respective 2 by 2 fiber optic coupler (36-1 . . .36-m). Each of the m fiber optic couplers (36-1-36-m) divides the lightfrom the respective light source (24-1 . . . 24-m) into m^(th)respective reference and sample light portions which propagate throughthe reference and sample branches of the m^(th) respective Michelsoninterferometer. The m respective reference light portions are coupledinto m respective reference delay optical fibers (1R . . . mR) woundaround the reference optical fiber stretcher 16R, and the m respectivesample light portions are coupled into m respective delay optical fibers(1S . . . mS) wound around the sample optical fiber stretcher 16S. Asdiscussed above with reference to FIG. 2A-FIG. 6B, the fiber opticcouplers (36-1 . . . 36-m) may have any splitting ratio but arepreferable 50/50 couplers. In the reference branch of each of therespective m Michelson interferometers, a portion of the lighttransmitted along reference delay optical fibers (1R . . . mR) isincident on a respective reference reflector (48R-1 . . . 48R-m). Thelocations of reflectors (48R-1 . . . 48R-m) may be pre-adjusted and setup at a distance to match path lengths of the reference and samplebranches of each of the m interferometers. A portion of the lightreaching each of the respective reference reflectors (48R-1 . . . 48R-m)is reflected back through the respective reference delay optical fiber(1R . . . mR) of the m reference optical fiber stretcher 16R and thenback through the respective fiber optic coupler (36-1 . . . 36-m) and isincident on a respective detector (26-1 . . . 26-m).

In the sample branch of each of the respective m Michelsoninterferometers of the first associated m optical fiber Interferometerembodiment 70A shown in FIG. 7A, a portion of the sample light portiontransmitted through sample delay optical fibers (1S . . . mS) isincident on a respective optical probe (44-1 . . . 44-m) which focusesor collimates light on the object 42 at respective measurement locations(L-1 . . . L-m). A portion of the light from each of the respectivelight sources (24-1 . . . 24-m) reaching the object 42 at respectivemeasurement locations (L-1 . . . L-m) is reflected back from each of theoptical interfaces of the object 42 back through the respective opticalprobes (44-1 . . . 44-m), the respective delay optical fiber (1S . . .mS) of the m sample optical fiber stretcher 16S, and the respectivefiber optic coupler (36-1 . . . 36-m), and is also incident onto therespective detector (26-1 . . . 26-m). Constructive interference willoccur between the two branches of each of the respective interferometerswhen the path lengths of the reference R and sample S branches are thesame and when they differ by an amount equal to distances betweenoptical interfaces in the object 42 at each of the respectivemeasurement locations (L-1 . . . L-m).

To summarize, each of the light sources (24-1 . . . 24-m) provide lightto a corresponding fiber optic coupler which divides the light intoreference and sample light portions, the reference light portion beingcoupled to the input ends of a distinct one of the m reference delayoptical fibers of the reference optical fiber stretcher 16R, and thesample light portion being coupled to the input ends of the samedistinct one of the m sample delay optical fibers of the sample opticalfiber stretcher 16S. A reference reflector is coupled to the output endof each of the distinct m reference delay optical fibers of thereference optical fiber stretcher 16R, where a portion of the referencelight portion is reflected back through the distinct one of the mreference delay optical fibers of the reference optical fiber stretcher,and back through the fiber optic coupler and into a detector. An opticalprobe is coupled to the output end of a distinct one of the m sampledelay optical fibers; the optical probe focuses the sample light portiononto a measurement location of an object, a portion of the sample lightportion being reflected back from one or more optical interfaces of theobject 42, back through the optical probe, back through the distinct oneof the m sample delay optical fibers, and back through the fiber opticcoupler and into the detector where interference is observed.

The first associated m optical fiber interferometer shown in FIG. 7Adoes not explicitly include a coherent light interferometer which isshown as the interferometers with suffix -1 shown in FIG. 2A-FIG. 6B.Any of the light sources (24-1 . . . 24-m) shown in FIG. 7A may be acoherent or a low coherence light source. Usually only one coherentlight source will be necessary to function with a built in real timecalibration and the coherent light source may be coupled to any one ofthe m interferometers. Also the detectors (26-1 . . . 26-m) may be usedto detect either coherent or low coherence light, and they are notdistinguished in the schematic of FIG. 7A. If the j^(th) interferometerwhere j is and integer and 1≤j≤m has a coherent light source 24-j at itsinput, then detector 26-j would detect the coherent light and the j^(th)interferometer would be used as the reference interferometer. In orderto properly use the jth interferometer as a coherent light referenceinterferometer, the jth optical probe 44-j and the jth measurementlocation L-j would be replaced with a reflector 48S-j (not shown) inFIG. 7A.

An example of an associated m optical fiber interferometer incorporatinga coherent light reference interferometer is shown in FIG. 7B. Thesecond associated m coupled fiber interferometer 70B shown in FIG. 7Bincorporates a coherent light interferometer which acts as a built inreference distance scale. Only the jth and mth interferometers are shownin FIG. 7B. The coherent light interferometer is shown as being theMichelson interferometer having the suffix -j, where j is an integerwith 1≤j≤m. Comparing the second associated m optical fiberinterferometer embodiment 70B to the first associated m optical fiberinterferometer embodiment 70A, light source 24-j has been replaced bycoherent light source 22-j, and detector 26-j has been replaced bycoherent light detector 25-j in the input side of fiber optic coupler36-j. In the sample branch, optical probe 44-j has been replaced bysample reflector 48S-j. With the exception of the jth light source, allthe other m−1 light sources are preferably low coherence light sourcesas shown in the schematic shown in FIG. 7B.

In general in the associated m optical fiber interferometers, each ofthe light sources provide coherent or low coherence light to acorresponding fiber optic coupler which divides the provided coherent orlow coherence light into reference and sample coherent or low coherencelight portions, which are coupled into the input ends of thecorresponding distinct one of the m reference delay and sample delayoptical fibers of the reference and sample delay optical fiberstretchers respectively.

In the preferred configuration, the jth light source, where j is aninteger with 1≤j≤m, is a coherent light source, and provides coherentlight of wavelength λ_(c) to the jth fiber optic coupler, where thecoherent light is divided into reference and sample coherent lightportions which are coupled to the input ends of the jth reference delayoptical fiber and the jth sample delay optical fiber respectively. Theremaining m−1 light sources are low coherence light sources providinglow coherence light to the corresponding remaining m−1 fiber opticcouplers which divide the low coherence light into m−1 reference andsample low coherence light portions being coupled to the input ends ofthe corresponding reference delay and sample delay optical fibersrespectively. A sample reflector is also coupled to the output end ofthe jth sample delay optical fiber where a portion of the samplecoherent light portion is reflected back through the jth sample delayoptical fiber and back through the jth fiber optic coupler and into thejth detector. Also, an optical probe is coupled to the output ends ofeach of the remaining m−1 sample delay optical fibers. The opticalprobes focus the sample light portion onto a measurement location of theobject, where a portion of the sample light portion being reflected backfrom the one or more optical interfaces of the object reflects backthrough the optical probe, back through the distinct one of theremaining m−1 sample delay optical fibers, and back through the fiberoptic coupler and into the corresponding detector.

The apparatus 70B shown in FIG. 7B thus comprises m associatedinterferometers where m is an integer greater than 1 with each of the massociated interferometers having a reference branch and a samplebranch. Each of the m associated interferometers further comprise acommon reference optical fiber stretcher 16R comprising a referenceouter surface 12R defining a reference perimeter, m reference delayoptical fibers (1R . . . mR) wound around the reference outer surface12R, and a reference actuator configured to temporally vary theperimeter of the reference outer surface 12R, and a common sampleoptical fiber stretcher 16S comprising a sample outer surface 12Sdefining a sample perimeter, m sample delay optical fibers (1S . . . mS)wound around the sample outer surface 12S, and a sample actuatorconfigured to temporally vary the perimeter of the sample outer surface23S. Each of the m associated interferometers also comprise a fiberoptic coupler (36-1 . . . 36 m) receiving coherent or low coherencelight from a coherent or low coherence light source and transmitting afirst portion of the coherent or low coherence light into an input endof a distinct one of the m reference delay optical fibers (1R . . . mR)and transmitting a second portion of the coherent or low coherence lightinto an input end of a distinct one of the m sample delay optical fibers(1S . . . mS); wherein the j^(th) fiber optic coupler 36-j, where j isan integer with 1≤j≤m, receives coherent light of wavelength λ_(c) fromthe coherent light source 22-j and transmits a first portion of thecoherent light into an input end of the j^(th) reference delay opticalfiber jR and transmits a second portion of the coherent light into aninput end of the j^(th) sample delay optical fiber jS. The remaining m−1fiber optic couplers receive low coherence light from a low coherencelight source and transmit a first portion of the low coherence lightinto an input end of the corresponding remaining one of the m−1reference delay optical fibers and transmit a second portion of the lowcoherence light into an input end of the corresponding remaining one ofthe m−1 sample delay optical fibers. A reference reflector (48R-1 . . .48R-m) is coupled to the output end of each of the distinct one of the mreference delay optical fibers (1R . . . mR) which reflects coherent orlow coherence light back through the distinct one of the m referencedelay optical fibers (1R . . . mR) and back through the correspondingdistinct one of the m fiber optic couplers (36-1 . . . 36-m) and into acorresponding detector. A sample reflector 48R-j is also coupled to theoutput end of the j^(th) sample delay optical fiber jR which reflectscoherent light back through the j^(th) sample delay optical fiber jS andback through the j^(th) fiber optic coupler 36-j and into the j^(th)detector 25-j. An optical probe is also coupled to the output end ofeach of the distinct one of the remaining m−1 sample delay opticalfibers, and configured to transmit low coherence light to a location onan object 42 comprising at least one optical interface, and receive lowcoherence light reflected back from the at least one optical interface,and transmit the reflected low coherence light back through the opticalprobe, back through the distinct one of the remaining m−1 sample delayoptical fibers, and back through the corresponding distinct one of them−1 fiber optic couplers and into the corresponding detector.

We now consider the case of the m associated optical fiberinterferometer shown FIG. 7B where m=4 and j=2 as an example. In thisexample, the common reference optical fiber stretcher 16R has fourreference delay optical fibers (1R, 2R, 3R and 4R) wound around itsreference outer surface 12R. The common sample optical fiber stretcher16S has four sample delay optical fibers (1S, 2S, 3S and 4S) woundaround its sample outer surface 12S. Fiber optic couplers 36-1, 36-3,and 36-4 receive low coherence light from corresponding low coherencelight sources 24-1, 24-3, and 24-4 respectively, and fiber optic coupler36-2 receives coherent light form coherent light source 22-2. Fiberoptic coupler 36-2 transmits a first portion of the coherent light intothe input end of reference delay optical fiber 2R and a second portionof the coherent light into the input end of sample delay optical fiber2S. The fiber optic couplers 26-1, 26-3, and 26-4 transmit a firstportion of the low coherence light into the input end of reference delayoptical fibers 1R, 3R, and 4R respectively, and transmit a secondportion of the low coherence light into the input end of sampled delayoptical fibers 1S, 3S, and 4S respectively. Reference reflectors 48R-1,48R-2, 48R-3, and 48R-4 are coupled to the output ends of referencedelay optical fibers 1R, 2R, 3R, and 4R respectively. A sample reflector48S-2 is coupled to the output end of sample delay optical fiber 2S,which reflect coherent light back through sample delay optical fiber 2Sand back through fiber optic coupler 36-2 and into detector 25-2.Optical probes 44-1, 44-3, and 44-4 are coupled to the output end ofsample delay optical fibers 1S, 3S, and 4S respectively. Low coherencelight is transmitted through optical probes 44-1, 44-3, and 44-4, and isincident on measurement locations L-1, L-3, and L-4 on the object 42 andreflected back into optical probes 44-1, 44-3, and 44-4 respectively,back through sample delay optical fibers 1S, 3S, and 4S respectively,and back through fiber optic couplers 36-1, 36-3, and 36-4 respectively,and into corresponding detectors 24-1, 24-3 and 24-4 respectively.

The associated m optical fiber interferometer configurations shown inFIGS. 7A-7B show configurations that have a matched pair of opticalfiber stretchers 16R and 16S having m optical fibers each in thereference and sample branches respectively of each of the associated moptical fiber interferometers which preferably operate in a push-pullmode. An associated m optical fiber interferometer may also beconstructed having only one common optical fiber stretcher having moptical fibers located in the reference branch of each of the mindependent interferometers similar in structure to the first associatedtwo optical fiber interferometer 20A shown in FIG. 2A. As in the case ofthe first associated two optical fiber interferometer 20A, the pathlengths of all the fibers in the sample branch 16S are required to beclosely matched to the path lengths of the fibers in each of therespective reference branches. This could be practical if the objectneeds to be located a long distance away from the instrument. It is alsoto be understood that m associated optical fiber autocorrelators may beconstructed having structures similar to the associated two and threeoptical fiber autocorrelators that are shown in FIG. 4A to FIG. 5B.Associated m hybrid optical fiber interferometers may also beconstructed with m coupled interferometers similar to those in FIG.6A-FIG. 6B.

Other components may be added to the m coupled fiber opticinterferometers 70A and 70B shown in FIG. 7A and FIG. 7B respectively.Optional visible light sources and optional wavelength divisionmultiplexers may be added as described with respect to FIG. 2A-FIG. 6Bto provide a visible indicator on the object at each measurementlocation. Also, circulators may be added to any of the interferometersand the respective detectors may be respective balanced detectors asdescribed above.

Additionally, multiple interferometers may share the same light source.In order to have n interferometers share the same light source, a 1 by noptical coupler would be used to couple a light from the light sourceinto the input puts of the n fiber optic couplers. All of theinterferometers may use the same center wavelength low coherence lightsources, or the light sources may be all different center wavelengths ora combination of the two. The optical probes could be mounted togetherin a line, grid, or circle, or be separate from each other at variouslocations of the object. This could be useful on a film production lineto be able to measure the film thickness at different downstreamlocations in the machine simultaneously. Different optical probes may beset up to simultaneously measure the object at the same location, oneprobe measuring from the front side, and one optical probe measuringfrom the back side of the object.

In some applications where ultimate accuracy is not required, it may notbe necessary to have a built in coherent light interferometer to providea built in distance scale using the j^(th) interferometer of the first mcoupled fiber interferometer 70A for calibrating the scanning distanceof each of the respective m interferometers in real time. In thesecases, an external calibration, a previous calibration, or extrareference reflections built into the optical probe may be sufficient.

An example alternative to supplying a coherent interferometer as a builtin reference scale is to focus one of the optical probes (44-1 . . .44-m) onto a known calibration sample instead of the object. Thecalibration sample, also called a reference test object, is preferably asample with multiple optical interfaces and known constant opticaldistances between each of its optical interfaces. FIG. 8A shows anexample reference test object 72 which could be used as the calibrationsample in an m coupled fiber interferometer as shown in FIG. 7A. Theexample reference test object 72 is comprised of four optical flatplates 75 having fixed air gaps 78 between them. In order to ensure thatthe gaps and optical thicknesses of the individual layers remainconstant, the reference test object 72 is preferably placed in anisothermal environment. When using the reference test object 72 as areal time interferometer calibration, the j^(th) optical probe 44-j,where 1≤j≤m, of the first associated m optical fiber interferometershown in FIG. 7A would be collimated or focused onto the reference testobject 72 instead of measurement location L-j on the object. Lowcoherence light that passes through the optical probe 44-j is reflectedoff of each of the front and back surfaces of the optical flat plates 75which are called reference surfaces 81 r . . . 88 r in order from leftto right respectively.

FIG. 8B. shows an interferogram 80 of a measured signal that would beobtained from the jth detector 26-j in the j^(th) interferometer of thereference test object 72 shown in FIG. 8A during operation of thecoupled interferometers. The x axis (horizontal axis) shows the depthscan distance in units of microns. The y axis shows the log intensity ofthe interference signal detected by the jth detector in relative units.Constructive interference occurs between the two branches of the j^(th)interferometer when the path lengths of the two branches of the j^(th)interferometer are the same and when they differ by an amount equal todistances between optical interfaces in the reference test object 72.Constructive interference occurs at the measured locations called 1^(st). . . 8^(th) reference peaks 81 . . . 88 in FIG. 8B, which correspond tothe locations of the 1^(st) . . . 8^(th) reference peaks 81 r . . . 88 rshown in FIG. 8A. Time based sampling would be utilized in this case anda calibration curve of distance versus time may be obtained from themeasured locations of the reference test object 72.

The associated interferometer embodiments shown in FIG. 2A-FIG. 7B whichshare one or two common optical fiber stretching elements are all shownas Michelson interferometer configurations. Other interferometerconfigurations are possible and FIG. 13 shows an example showing analternative associated two optical fiber interferometer 20D in aMach-Zehnder configuration. Most of the components are the same as thatshown in FIG. 4B and components sharing the same numbers are the same asin FIG. 4B and have the same function. In the Mach-Zehnderconfiguration, light only is transmitted through the reference andsample delay optical fibers of the respective reference and sampleoptical fiber stretchers 10R and 10S once, and the reflectors 48R-1,48S-1, 48R-2 and 48S-2 shown in FIG. 4B are replaced with optical fibercouplers 36 b-1 and 36 b-2, which recombine the light transmitted alongreference delay optical fibers 1R, 1S, 2R, and 2S to cause interference.As described above, the respective reference and sample optical fiberstretchers 10R and 10S simultaneously vary the optical path length oflight of the same wavelength transmitted along the first reference andsecond reference delay optical fibers 1R and 2R by the same amount andthe optical path length of light of the same wavelength transmittedalong the first sample and second sample delay optical fibers 1S and 2Sby the same amount. The reference and sample optical fiber stretchersalso have uniform reference and sample outer surfaces which definereference and sample perimeters and are constructed the same way asdescribed above with reference to the earlier Figures.

The details of the alternative Mach Zehnder associated two optical fiberinterferometer 20D shown in FIG. 13 are described below. Interferometer20D is a dual interferometer comprised of a coherent lightinterferometer, the components of which having suffix 1, coupledtogether with a low coherence light interferometer in an autocorrelationmode, the components of which having suffix 2. In the coherent lightinterferometer, coherent light of wavelength λ_(c) from coherent lightsource 22-1 is coupled into fiber optic coupler 36-1, which divides thecoherent light into reference and sample coherent light portionstransmitted along the first reference delay fiber 1R and the sampledelay fiber 1S, which are wound around the reference and sample opticalfiber stretchers 10R and 10S respectively. After passing through thereference and sample optical fiber stretchers 10R and 10S, the referenceand sample coherent light portions transmitted along first referencedelay fiber 1R and first sample delay fiber 1S are recombined as theypass through first output fiber optic coupler 36 b-1. This recombinedinterfering coherent light is input into coherent light detector 25-1.First low coherence light of center wavelength λ₁ from first lowcoherence light source 24-2 is input into the first port of a firstprimary circulator 38 b-2, and is transmitted through the second port ofthe first primary circulator 38 b-2, and is coupled into a first opticalprobe 44-2 and focused onto a first measurement location L-2 of anobject 42. A portion of the first low coherence light is reflected backfrom one or more optical interfaces of the object 42 and back throughthe first optical probe 44-2, back through the second port of theprimary circulator 38 b-2, and through its third port into a secondfiber optic coupler 36-2, where it is divided into first reference andfirst sample low coherence light portions. The first reference and thefirst sample low coherence light portions are coupled to the input endsof the second reference delay optical fiber 2R and the second sampledelay optical fiber 2S, which are wound around the reference and sampleoptical fiber stretchers 10R and 10S respectively. After passing throughthe reference and sample optical fiber stretchers 10R and 10S, thereference and sample low coherence light portions transmitted alongsecond reference delay fiber 2R and second sample delay fiber 2S arerecombined as they pass through second output fiber optic coupler 36b-2. This recombined interfering light is input into a low coherencelight detector shown as a first balanced detector 28-2. In the firstbalanced detector 28-2, the two output branches of second output fiberoptic coupler 36 b-2 are connected to the two inputs of the firstbalanced detector 28-2. First balanced detector 28-2 may be a standardfirst low coherence light detector 26-2 which uses only one of theoutput optical fibers of second output fiber optic coupler 36 b-2. Aswith the Michelson interferometer configurations, it is preferred thatthe reference and sample optical fiber stretchers are operated in thepush pull mode.

Although FIG. 13 is shown as having two optical fiber coupledinterferometers it is to be understood that there could be three, four,or in general m coupled interferometers, with all of the low coherenceinterferometers being configured as autocorrelators input intoMach-Zehnder interferometers such as that shown as suffix 2 in FIG. 13.Each of the m coupled Mach-Zehnder interferometers would share commonreference and sample fiber optic stretchers.

The present invention also includes methods of utilizing the variousapparatuses described in the Figures. FIG. 10 shows a flow chartdepicting methods 100 of measuring one or more physical properties of anobject using apparatuses of the present invention. The first step 101may be to provide at least two coupled optical fiber interferometerswhich share a common reference optical fiber stretcher in the referencebranches of the coupled fiber optic interferometers, and also share acommon sample optical fiber stretcher in the sample branches of thecoupled fiber optic interferometers as described with reference to theembodiments shown in FIG. 2B-FIG. 7B. The common reference optical fiberstretcher has a reference outer surface defining a uniform referenceperimeter with at least two reference delay optical fibers woundtogether around its reference outer surface in a single layer and alsocomprises a reference actuator for temporally varying its referenceperimeter. The common sample optical fiber stretcher has a sample outersurface defining a uniform sample perimeter and at least two sampledelay optical fibers wound together around its sample outer surface in asingle layer and also comprises a sample actuator for temporally varyingits sample perimeter. In the preferred embodiment, each of the referencedelay optical fibers are interleaved with each other as they are woundtogether around the reference outer surface of the reference opticalfiber stretcher in a single layer, and each of the sample delay opticalfibers are interleaved with each other as they are wound together aroundthe sample outer surface of the sample optical fiber stretcher in asingle layer. Also, the lengths of the reference delay optical fiberswhich are interleaved and wound together around the reference outersurface of the reference optical fiber stretcher and the lengths of thesample delay optical fibers being interleaved and wound together aroundthe sample outer surface of the sample optical fiber stretcher arepreferably the same. The two or more coupled fiber optic interferometersmay be in either the standard Michelson configuration, autocorrelatorconfiguration or hybrid configuration as described with reference toFIG. 2B-7B. Alternatively, the common sample optical fiber stretcher maybe replaced with optical fibers of the equivalent length of those in thecommon reference optical fiber stretcher as shown in the embodimentshown in FIG. 2A.

In Step 101 coherent light of wavelength λ_(c) from a coherent lightsource is optionally provided to a provided first fiber optic couplerwhich divides the optional coherent light into reference and samplecoherent light portions. The reference and sample coherent lightportions are coupled to the input ends of the first reference delayoptical fiber and the first sample delay optical fiber respectively.

In Step 101, first low coherence light of center wavelength λ₁ is alsoprovided from a first low coherence light source to a second fiber opticcoupler which divides the first low coherence light into first referenceand first sample low coherence light portions. The first reference andfirst sample low coherence light portions are coupled to the input endsof the second reference delay optical fiber and the second sample delayoptical fiber respectively. A second reference reflector coupled to theoutput end of the second reference delay optical fiber is also providedin Step 101. A portion of the first reference low coherence lightportion reaching the second reference reflector is reflected backthrough the second reference delay optical fiber and back through thesecond fiber optic coupler into a first low coherence light detector. Afirst optical probe is also provided which is coupled to the output endof the second sample delay optical fiber.

In Step 102, the reference actuator of the reference optical fiberstretcher and the sample actuator of the sample optical fiber stretcherare continually actuated with periodic reference and sample waveformsrespectively having the same period. The periodic reference waveformapplied to the reference actuator causes the optical path length oflight of the same wavelength transmitted along the first reference delayoptical fiber and the second reference delay optical fiber tosimultaneously vary by the same amount with the period of the referencewaveform, while the periodic sample waveform applied to the sampleactuator causes the optical path length of light of the same wavelengthtransmitted along the first sample delay optical fiber and the secondsample delay optical fiber to simultaneously vary by the same amountwith the same period. It is preferred that the periodic reference andsample waveforms are out of phase with each other so that a periodicoptical path difference is produced between the reference and samplebranches of the two or more coupled fiber optic interferometers. It isfurther preferred that the periodic optical path delay between thereference and sample branches of the two or more coupled fiber opticinterferometers varies by a distance sufficient to determine one or morephysical properties of the object. The preferred embodiment uses thepush-pull mode of operation as shown in FIG. 9. When operating in thepush-pull mode, the relative optical delay between the sample andreference branches of the interferometer is alternately monotonicallyincreased and monotonically decreased over the distance sufficient todetermine the one of more physical properties of the object. Also, thesample branch is monotonically increasing in optical path while thereference branch is decreasing in optical path and vice versa.

In Step 103, the reference distance scale is set up. In the preferredembodiment, one of the coupled fiber interferometers set up during Step101 provides coherent light of wavelength λ_(c) from a coherent lightsource to a first fiber optic couple where the coherent light is dividedinto reference and sample coherent light portions. The reference andsample coherent light portions are coupled to the input ends of a firstreference delay optical fiber and a first sample delay optical fiberrespectively. First reference and first sample reflectors are alsoprovided which are coupled to the output ends of the first referencedelay and first sample delay optical fibers respectively. The firstreference reflector reflects a portion of the reference coherent lightportion back through the first reference delay optical fiber and backthrough the first fiber optic coupler into a provided coherent lightdetector, and the first sample reflector reflects a portion of thesample coherent light portion back through the first sample delayoptical fiber, and back through the first fiber optic coupler, and intothe coherent light detector, and the coherent light interference signalis detected. Since the wavelength of the coherent light source is aconstant, the coherent light interference signal may be used as aprecise distance scale.

FIG. 11 shows an example coherent light interferogram 92 for a 1310 nmlaser diode as a function of delay in microns. The observed signal issinusoidal with a constant period as a function of scan distance andwill remain sinusoidal over the coherence length of the laser which ismany millimeters or meters in path length difference. The interferogram92 shown in FIG. 11 has a period of 655 nm (λc/2) and zero crossingsoccur every 327.5 nm (λc/4). The coherent light interferometer signalmay be used to trigger data acquisition of the low coherence lightinterferometer signal at constant distance intervals. Usually the zerocrossings or a subset of the zero crossings are used for this purpose.

In Step 104, one or more optical probes are set up, focused, and alignedwith an object so that interferometric signals from each of the opticalinterfaces in the object may be observed. The first optical probecarrying the first sample low coherence light portion is focused andaligned onto a first measurement location of the object. A portion ofthe first sample low coherence light portion is reflected back from oneor more optical interfaces of the object, back through the first opticalprobe, back through the second sample delay optical fiber, and backthrough the second fiber optic coupler and into the first low coherencelight detector. The optical probe is aligned by observing the intensityof the peaks in the low coherence light interference signal (see FIG.12) and adjusting the orientation of the optical probe so that theintensity of the peaks in the interference signal is a maximum.

FIG. 12 shows an example interferogram 94 for an object (solid line)along with the optical path difference between the two branches of theinterferometer (dashed line) as a function of cumulative scan distance.In this case a standard 1 mm thick microscope slide having an index ofrefraction n=1.5 is the object. FIG. 12 also shows the optical pathdifference between the two branches of the interferometer as a functionof cumulative scan distance. The directional arrows 96 and 97 at the topof the Figure indicate the depth scan direction. Arrow 96 indicates theforward depth scan direction in which the scan direction is from thefront to the back of the object, and arrow 97 indicates the reverse scandirection in which the scan direction is from the back to the front ofthe object. The scan distance for the case shown in the interferogram 94in FIG. 12 is 4000 μm. The interferogram peaks indicative of the frontand back surface of the microscope slide are labelled F and Brespectively in FIG. 12. The interferogram shown is that for the firstlow coherence light detector after being processed by a demodulating logamplifier. The measured distances between the front F and back Bsurfaces of the object is indicated by the bidirectional arrows labellednt in FIG. 12. Data for five successive scans are shown.

In Step 105, the coherent light and low coherence light interferogramsare collected by the coherent light detector and the first low coherencelight detector. In preferred embodiments, the coherent lightinterferometer data is used to trigger data acquisition of the lowcoherence light detector signal as described above. The data is obtainedcontinuously and may be stored in a computer memory or processed in realtime.

In Step 106, the interferogram peak locations are determined withrespect to scan distance and sorted into bins defining which surface ofthe object that they arise from. Interpolation algorithms may be appliedto the peaks to determine the exact center of the peak location as afunction of scan distance. The forward and reverse scan directions mayalso be sorted to correct the ordering of the peaks.

In step 107, one or more physical properties of the object aredetermined. In the case of the glass microscope slide data shown in FIG.12, the optical thickness nt of the glass microscope slide may bedirectly determined by the difference between the distances between thesets of peaks F and B respectively indicating the front and back opticalsurfaces of the sample. Five successive measurements of the opticalthickness nt are shown in FIG. 12 as indicated by the distance betweenthe double arrow regions labelled nt.

Other examples of physical properties of an object that may be measuredinclude optical distances between adjacent layers of the object,distance from a reference surface to a surface of the object, index ofrefraction, and physical thickness. Environmental and spatial dependentproperties may also be determined by ramping one parameter or scanningover the surface of the object during measurement. As an example, thetemperature coefficient of index of refraction and the thermalcoefficient of expansion may be measured by varying the temperaturetogether with the interferometer measurements.

The invention has been described in detail with particular reference tocertain example embodiments thereof, but it will be rather apparent tothose skilled in the art that the foregoing detailed disclosure isintended to be presented by way of example only, and is not limiting.Various alterations, improvements, and modifications will occur to thoseskilled in the art, though not expressly stated herein. Thesealterations, improvements, and modifications are intended to besuggested hereby, and are within the spirit and scope of the invention.Additionally, the recited order of processing elements or sequences, orthe use of numbers, letters, or other designations therefore, is notintended to limit the claimed processes to any order except as may bespecified in the claims.

PARTS LIST

-   L-1 Measurement Location-   L-2 Measurement Location-   L-3 Measurement Location-   L-m Measurement Location-   mR m^(th) Reference Delay Optical Fiber-   mS m^(th) Sample Delay Optical Fiber-   R Reference Branch-   S Sample Branch-   1 First Delay Optical Fiber-   1R First Reference Delay Optical Fiber-   1S First Sample Delay Optical Fiber-   1S-P First Sample Path Matching Optical Fiber-   2 Second Delay Optical Fiber-   2R Second Reference Delay Optical Fiber-   2S Second Sample Delay Optical Fiber-   2S-P Second Sample Path Matching Optical Fiber-   3 Third Delay Optical Fiber-   3R Third Reference Delay Optical Fiber-   3 Sa Third Sample Delay Optical Fiber-   4 Fourth Delay Optical Fiber-   5 Piezoelectric Cylinder-   6 Piezoelectric Cylinder Outer Surface-   7 Piezoelectric Cylinder Inner Surface-   10 Optical Fiber Stretcher-   10R Reference Optical Fiber Stretcher-   10S Sample Optical Fiber Stretcher-   12R Reference Outer Surface-   12S Sample Outer Surface-   13 Optical Fiber Stretcher-   13R Reference Optical Fiber Stretcher-   13S Sample Optical Fiber Stretcher-   14 Optical Fiber Stretcher-   16R Reference Optical Fiber Stretcher-   16S Sample Optical Fiber Stretcher-   20A Associated Two Optical Fiber Interferometer-   20B Associated Two Optical Fiber Interferometer-   20C Associated Two Optical Fiber Interferometer-   20D Alternative Associated Two Optical Fiber Interferometer-   22-1 Coherent Light Source-   24-1 Light Source-   24-2 Light Source-   24-3 Light Source-   24-m Light Source-   25-1 Coherent Light Detector-   26-1 Detector-   26-2 Detector-   26-3 Detector-   26-m Detector-   28-2 First Balanced Detector-   28-3 Second Balanced Detector-   30A Associated Three Optical Fiber Interferometer-   30B Associated Three Optical Fiber Interferometer-   32-2 First Visible Light Source-   32-3 Second Visible Light Source-   34-2 First Wavelength Division Multiplexer-   34-3 Second Wavelength Division Multiplexer-   36-1 First Fiber Optic Coupler-   36-2 Second Fiber Optic Coupler-   36-3 Third Fiber Optic Coupler-   36-m mth Fiber Optic Coupler-   36 b-1 First Output Fiber Optic Coupler-   36 b-2 Second Output Fiber Optic Coupler-   38-2 First Circulator-   38-3 Second Circulator-   38 b-2 First Primary Circulator-   38 b-3 Second Primary Circulator-   40A Associated Two Optical Fiber Autocorrelator-   40B Associated Two Optical Fiber Autocorrelator-   42 Object-   44-1 Optical Probe-   44-2 Optical Probe-   44-3 Optical Probe-   44-m Optical Probe-   48R-1 First Reference Reflector-   48R-2 Second Reference Reflector-   48R-3 Third Reference Reflector-   48R-m mth Reference Reflector-   48S-1 First Sample Reflector-   48S-2 Second Sample Reflector-   48S-3 Third Sample Reflector-   50A Associated Three Optical Fiber Autocorrelator-   50B Associated Three Optical Fiber Autocorrelator-   60A Associated Three Optical Fiber Hybrid Interferometer-   60B Associated Three Optical Fiber Hybrid Interferometer-   70A Associated m Optical Fiber Interferometer-   70B Associated m Optical Fiber Interferometer-   72 Reference Test Object-   75 Optical Flat Plate-   78 Air Gap-   80 Reference Test Object Interferogram-   81 1^(st) Reference Peak-   81 r 1^(st) Reference Surface-   82 2^(nd) Reference Peak-   82 r 2^(nd) Reference Surface-   83 3^(rd) Reference Peak-   83 r 3^(rd) Reference Surface-   84 4^(th) Reference Peak-   84 r 4^(th) Reference Surface-   85 5th Reference Peak-   85 r 5^(th) Reference Surface-   86 6th Reference Peak-   86 r 6^(th) Reference Surface-   87 7th Reference Peak-   87 r 7^(th) Reference Surface-   88 8^(th) Reference Peak-   88 r 8^(th) Reference Surface-   90 Actuator Voltage Versus Time Graph-   92 Coherent Light Interferogram-   94 Device Under Test Interferogram-   96 Forward Depth Scan-   97 Reverse Depth Scan-   100 Flow Chart-   101 Step-   102 Step-   103 Step-   104 Step-   105 Step-   106 Step-   107 Step

We claim:
 1. A method of measuring at least one physical property of anobject using m associated interferometers where m is an integer greaterthan 1, each of the m associated interferometers comprising: a referencebranch and a sample branch; a common reference optical fiber stretcherhaving an outer common reference surface with a uniform perimeter and areference actuator operable to temporally vary the uniform perimeter ofthe common reference surface, and m reference delay optical fibers woundtogether around the outer common reference surface; a common sampleoptical fiber stretcher having an outer common sample surface with auniform perimeter and a sample actuator operable to temporally vary theuniform perimeter of the common sample surface, and m sample delayoptical fibers wound together around the outer common surface; a lightsource providing light to a fiber optic coupler; and an optical probecoupled to an output end of at least a distinct one of m−1 sample delayoptical fibers of the m sample delay fibers; and the method comprising:a) dividing the light from the light source into reference and samplelight portions, and coupling the reference light portion to the inputend of a distinct one of the m reference delay optical fibers of thecommon reference optical fiber stretcher and coupling the sample lightportion to an input end of the distinct one of the m sample delayoptical fibers of the common sample optical fiber stretcher; b) couplinga reference reflector to the output end of the distinct one of the mreference delay optical fibers of the common reference optical fiberstretcher, reflecting a portion of the reference light portion backthrough the distinct one of the m reference delay optical fibers of thecommon reference optical fiber stretcher and back through the fiberoptic coupler and into a detector; c) focusing the sample light portiononto a measurement location of the object, reflecting a portion of thesample light portion back from at least one optical interface of theobject, back through the optical probe, back through the distinct one ofthe m−1 sample delay optical fibers of the m sample delay fibers andback through the fiber optic coupler and into the detector; d) actuatingthe reference actuator with a periodic reference waveform tosimultaneously vary the optical path length of each of the m referencedelay optical fibers by the same amount while actuating the sampleactuator with a periodic sample waveform to simultaneously vary theoptical path length of each of the m sample delay optical fibers by thesame amount; e) collecting the interference signals induced in each ofthe m respective detectors; and f) analyzing the interference signals todetermine the value of at least one physical property of the object ateach of the respective measurement locations.
 2. The method of claim 1,wherein the m^(th) light source is a coherent light source providingcoherent light of wavelength λ_(c) and a reference reflector is coupledto the output end of the m^(th) sample delay optical fiber of the commonsample optical fiber stretcher, operable to reflect a portion of thesample light portion back through the m^(th) sample delay optical fiberof the common sample optical fiber stretcher and back through the fiberoptic coupler and into a detector.
 3. The method of claim 2, wherein theremaining m−1 light sources are low coherence light sources.
 4. Themethod of claim 3, wherein the low coherence light sources all have thesame center wavelength equal to λ_(c).
 5. The method of claim 1, whereinthe reference and sample optical fiber stretchers are comprised ofpiezoelectric cylinders having the same perimeter and are operable asthe respective reference and sample actuators.
 6. The method of claim 1,wherein m reference delay optical fibers are interleaved with each otheraround the outer surface of the common reference optical fiber stretcherin a single layer; the m sample delay optical fibers are interleavedwith each other the outer surface of the common sample optical fiberstretcher in a single layer, and wherein the lengths of the m referencedelay optical fibers and the lengths of the m sample delay opticalfibers are the same.
 7. The method of claim 6, wherein the outersurfaces of all adjacent interleaved optical fibers are in contact witheach other.
 8. The method of claim 1, wherein the periodic reference andsample waveforms have the same period and the reference and sampleactuators are operated in a push-pull mode.
 9. A method of measuring atleast one physical property of an object, the method comprising: a)providing reference and sample optical fiber stretchers having outersurfaces with uniform perimeters comprising reference and sampleactuators respectively for temporally varying the perimeters of thereference and sample optical fiber stretchers respectively, thereference optical fiber stretcher having first and second referencedelay optical fibers wound together around the outer surface of thereference optical fiber stretcher in a single layer, and the sampleoptical fiber stretcher having first and second sample delay opticalfibers wound together around the outer surface of the sample opticalfiber stretcher in a single layer; b) providing coherent light ofwavelength from a coherent light source to a first fiber optic coupler,dividing the coherent light into reference and sample coherent lightportions and coupling the reference and sample coherent light portionsto the input ends of the first reference delay optical fiber and thefirst sample delay optical fiber respectively; c) providing firstreference and sample reflectors coupled to the output ends of the firstreference delay and first sample delay optical fibers respectively,reflecting a portion of the reference coherent light portion backthrough the first reference delay optical fiber and back through thefirst fiber optic coupler into a coherent light detector; and reflectinga portion of the sample coherent light portion back through the firstsample delay optical fiber and back through the first fiber opticcoupler and into the coherent light detector; d) providing first lowcoherence light of center wavelength λ₁ from a first low coherence lightsource to a first port of a first primary optical circulator, couplingthe first low coherence light coming out of the second port of the firstprimary optical circulator into a first optical probe, focusing thefirst low coherence light onto a first measurement location of theobject, reflecting a portion of the first low coherence light portionback from one or more optical interfaces of the object, back through thefirst optical probe, back through the second port of the primary opticalcirculator, coupling the reflected first low coherence light portionfrom the object passing through the third port of the first primaryoptical circulator into a second fiber optic coupler; dividing thereflected first low coherence light portion from the object into firstreference and first sample low coherence light portions and coupling thefirst reference and first sample low coherence light portions to theinput ends of the second reference delay optical fiber and the secondsample delay optical fiber respectively; e) providing second referenceand second sample reflectors coupled to the output ends of the secondreference delay and second sample delay optical fibers respectively,reflecting a portion of first reference low coherence light portion backthrough the second reference delay optical fiber and back through thesecond fiber optic coupler into a first low coherence light detector;and reflecting a portion of the first sample low coherence light portionback through the second sample delay optical fiber and back through thesecond fiber optic coupler and into the first low coherence lightdetector; f) actuating the reference actuator with a periodic referencewaveform to simultaneously vary the optical path length of the firstreference delay optical fiber and the second reference delay opticalfiber by the same amount while actuating the sample actuator with aperiodic sample waveform to simultaneously vary the optical path lengthof the first sample delay optical fiber and the second sample delayoptical fiber by the same amount; g) collecting a coherent lightinterference signal induced in the coherent light detector whilecollecting a first low coherence light interference signal in the firstlow coherence light detector and using the coherent light interferencesignal as a reference distance scale for the first low coherence lightinterference signal to determine the locations of peaks occurring in thefirst low coherence light interference signal; and h) determining thevalue of the at least one physical property of the object at the firstmeasurement location from the measured locations of peaks in the firstlow coherence light interference signal.
 10. The method of claim 9,wherein the reference and sample optical fiber stretchers are comprisedof piezoelectric cylinders having the same perimeter and are operable asthe respective reference and sample actuators.
 11. The method of claim9, wherein the first and second reference delay optical fibers areinterleaved with each other around the outer surface of the referenceoptical fiber stretcher in a single layer; the first and second sampledelay optical fibers are interleaved with each other around the outersurface of the sample optical fiber stretcher in a single layer, and thelengths of the first and second reference delay optical fibers and thelengths of the first and second sample delay optical fibers are thesame.
 12. The method of claim 11, wherein the outer surfaces of alladjacent interleaved optical fibers are in contact with each other. 13.The method of claim 12, wherein the periodic reference and samplewaveforms have the same period and the reference and sample actuatorsare operated in a push-pull mode of operation.
 14. The method of claim9, further comprising providing a first circulator having its first portbeing coupled to the output of the third port of the first primaryoptical circulator, its second port being coupled to the second fiberoptic coupler and its third port being coupled to an input port of afirst balanced detector operable as the first low coherence lightdetector, the other input port of the balanced detector receiving theportion of the light reflected back from the object at the firstmeasurement location combined with the portion of the light reflectedback from the second reference reflector that passes back through thesecond fiber optic coupler.
 15. The method of claim 9, where λ_(c) isequal to λ₁.
 16. The method of claim 9, wherein the reference opticalfiber stretcher includes a third reference delay optical fiber, thefirst, second and third reference delay optical fibers being woundtogether around the outer surface of the reference optical fiberstretcher in a single layer, and the step of actuating the referenceactuator with a periodic reference waveform simultaneously varies theoptical path length of the first, second and third reference delayoptical fibers by the same amount; and the sample optical fiberstretcher includes a third sample delay optical fiber, the first, secondand third sample delay optical fibers being wound together around theouter surface of the sample optical fiber stretcher in a single layer,and the step of actuating the sample actuator with a periodic samplewaveform simultaneously varies the optical path length of the first,second and third sample delay optical fibers by the same amount, andwherein the method further comprises: i) providing second low coherencelight of center wavelength λ₂ from a second low coherence light sourceto a first port of a second primary optical circulator, coupling thesecond low coherence light coming out of the second port of the secondprimary optical circulator into a second optical probe, focusing thesecond low coherence light onto a second measurement location of theobject, reflecting a portion of the second low coherence light portionback from one or more optical interfaces of the object, back through thesecond optical probe, back through the second port of the second primaryoptical circulator, coupling the reflected second low coherence lightportion from the object passing through the third port of the secondprimary optical circulator into a provided third fiber optic coupler;dividing the reflected second low coherence light portion from theobject into second reference and second sample low coherence lightportions and coupling the second reference and second sample lowcoherence light portions to the input ends of the third reference delayoptical fiber and the third sample delay optical fiber respectively; j)providing third reference and third sample reflectors coupled to theoutput ends of the third reference delay and third sample delay opticalfibers respectively, reflecting a portion of second reference lowcoherence light portion back through the third reference delay opticalfiber and back through the third fiber optic coupler into a second lowcoherence light detector; and reflecting a portion of the second samplelow coherence light portion back through the third sample delay opticalfiber and back through the third fiber optic coupler and into the secondlow coherence light detector; k) collecting a second low coherence lightinterference signal in the second low coherence light detector whilecollecting the coherent light interference signal and using the coherentlight interference signal as a reference distance scale for the secondlow coherence light interference signal to determine the locations ofpeaks occurring in the second low coherence light interference signal;and l) determining the value of the at least one physical property ofthe object at the second measurement location from the measuredlocations of peaks in the second low coherence light interferencesignal.
 17. The method of claim 9, further comprising providing a firstcirculator having its first port being coupled to the first lowcoherence light source, its second port being coupled to the secondfiber optic coupler and its third port being coupled to an input port ofa first balanced detector operable as the first low coherence lightdetector, the other input port of the balanced detector receiving theportion of the light reflected back from the object at the firstmeasurement location combined with the portion of the light reflectedback from the second reference reflector that passes back through thesecond fiber optic coupler, and a second circulator having its firstport being coupled to the second low coherence light source, its secondport being coupled to the third fiber optic coupler and its third portbeing coupled to an input port of a second balanced detector operable asthe second low coherence light detector, the other input port of thesecond balanced detector receiving the portion of the light reflectedback from the object at the second measurement location combined withthe portion of the light reflected back from the third referencereflector that passes back through the third fiber optic coupler.
 18. Amethod of measuring at least one physical property of an object, themethod comprising: a) providing reference and sample optical fiberstretchers having outer surfaces with uniform perimeters comprisingreference and sample actuators respectively for temporally varying theperimeters of the reference and sample optical fiber stretchersrespectively, the reference optical fiber stretcher having first andsecond reference delay optical fibers wound together around the outersurface of the reference optical fiber stretcher in a single layer, andthe sample optical fiber stretcher having first and second sample delayoptical fibers wound together around the outer surface of the sampleoptical fiber stretcher in a single layer; b) providing coherent lightof wavelength λ_(c) from a coherent light source to a first fiber opticcoupler, dividing the coherent light into reference and sample coherentlight portions and coupling the reference and sample coherent lightportions to the input ends of the first reference delay optical fiberand the first sample delay optical fiber respectively; c) providing afirst output fiber optic coupler coupled to a coherent light detectorcoupled to the output ends of the first reference delay and first sampledelay optical fibers respectively; d) providing first low coherencelight of center wavelength λ₁ from a first low coherence light source toa first port of a first primary optical circulator, coupling the firstlow coherence light coming out of the second port of the first primaryoptical circulator into a first optical probe, focusing the first lowcoherence light onto a first measurement location of the object,reflecting a portion of the first low coherence light portion back fromone or more optical interfaces of the object, back through the firstoptical probe, back through the second port of the primary opticalcirculator, coupling the reflected first low coherence light portionfrom the object passing through the third port of the first primaryoptical circulator into a second fiber optic coupler; dividing thereflected first low coherence light portion from the object into firstreference and first sample low coherence light portions and coupling thefirst reference and first sample low coherence light portions to theinput ends of the second reference delay optical fiber and the secondsample delay optical fiber respectively; e) providing a second outputfiber optic coupler coupled to a first low coherence light detectorcoupled to the output ends of the second reference delay and secondsample delay optical fibers respectively; f) actuating the referenceactuator with a periodic reference waveform to simultaneously vary theoptical path length of the first reference delay optical fiber and thesecond reference delay optical fiber by the same amount while actuatingthe sample actuator with a periodic sample waveform to simultaneouslyvary the optical path length of the first sample delay optical fiber andthe second sample delay optical fiber by the same amount; g) collectinga coherent light interference signal induced in the coherent lightdetector while collecting a first low coherence light interferencesignal in the first low coherence light detector and using the coherentlight interference signal as a reference distance scale for the firstlow coherence light interference signal to determine the locations ofpeaks occurring in the first low coherence light interference signal;and h) determining the value of the at least one physical property ofthe object at the first measurement location from the measured locationsof peaks in the first low coherence light interference signal.
 19. Themethod of claim 18, wherein the first low coherence light detector is abalanced detector.